HI Frrst,
You have quite a few questions here, so I’ll do my best to consolidate some of the information for which you were searching. I don’t mind helping to gather the information for you!
[quote]I am making a king bed for me and my wife
decided we like a firm bed
she also is very adamant that she wants a 3 inch memory foam topper
I am leaning towards a Aerus + 3 inch topper for 399$ which is quite spendy, if anyone has any experience with this or other high density cooling memory foams I would really appreciate some feed back on what works and if it is worth the price.[/quote]
There is more information about choosing a topper in post #2 here and the topper guidelines it links to which along with a conversation with a reliable and knowledgeable supplier (that can provide you with good information about how their toppers compare to each other or to other toppers they are familiar with that are available on the market) can help you use your sleeping experience as a reference point and guideline to help you choose the type, thickness, and firmness for a topper that has the least possible risk and the best chance for success. It also includes a link to a list of some of the better online sources for toppers I’m aware of and a link to the online suppliers that have good exchange/return policies as well (just in case the topper you choose doesn’t turn out as well as you hoped for).
I don’t have any specific suggestions or recommendations because you are the only one that can feel what you feel on a mattress/topper combination and there are too many unknowns, variables, and personal preferences involved for anyone to be able to predict or make a specific suggestion about which topper would be the best “match” for both you and the mattress you are using it on in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences) based on specs (either yours or a mattress) or “theory at a distance” that can possibly be more accurate than your own personal testing or sleeping experience (see mattress firmness/comfort levels in post #2 here).
If a mattress/topper combination turns out to be a good “match” for you in terms of "comfort and PPP then it also has the advantage of being able to replace just the topper without replacing the entire mattress if it softens or breaks down before the upper foam layers in the mattress (which is very likely because the upper layers of a sleeping system tend to soften or break down before the deeper layers) or if your needs or preferences change over time and a topper can also help extend the useful life of the mattress underneath it as well.
Regarding cooling memory foam claims, you can read more about phase change materials in post #9 here and at the end of post #4 here) and you can read more about the various different types of gel foams in post #2 here. In general terms gel foams will tend to have a temporary effect on temperature while you are first going to sleep until temperatures equalize but have less effect on temperature regulation throughout the course of the night.
There is more information in post #9 here about the different ways that one mattress can “match” or “approximate” another one. Every layer and component in a mattress (including the cover and any quilting materials) will affect the feel and performance of every other layer and component and the mattress “as a whole” so unless you are able to find another mattress that uses exactly the same type of materials, components, cover and quilting, layer thicknesses, layer firmnesses, and overall design (which would be fairly unlikely) then there really isn’t a reliable way to match one mattress to another one in terms of “comfort”, firmness, and PPP (Posture and alignment, Pressure relief, and your Personal preferences) based on the specifications of the mattresses (even assuming that you can find out all the specifications you would need for both mattresses you are comparing in the first place). And since you are using a latex core (a high quality material that is very different form the polyfoam core used in most memory foam beds), you’re already creating quite a bit of a different product from the start.
[quote]Right now I am considering two 3" 44 ILD layers from sleep on latex. Is 44 ILD directly below 3 inches of 5lb memory foam going to be too firm (I know this is relative, but I guess I am asking would it be much more firm than a ‘firm mattress we would find in a store)’.
Is 6 inches under 3 inches of memory foam enough to offer a good sleep surface or should I consider one to two inch softer latex transition layer?[/quote]
There are also no “standard” definitions or consensus of opinions for firmness ratings and different manufacturers can rate their mattresses very differently than others so a mattress that one manufacturer rates as being a specific firmness could be rated very differently by another manufacturer. Different people can also have very different perceptions of firmness and softness compared to others as well and a mattress that feels firm for one person can feel like “medium” for someone else or even “soft” for someone else (or vice versa) depending on their body type, sleeping style, physiology, their frame of reference based on what they are used to, and their individual sensitivity and perceptions. There are also different types of firmness and softness that different people may be sensitive to that can affect how they “rate” a mattress as well (see post #15 here) so different people can also have very different opinions on how two mattresses compare in terms of firmness and some people may rate one mattress as being firmer than another and someone else may rate them the other way around. This is all relative and very subjective and is as much an art as a science.
And of course there is absolutely no way I would be able to predict what is a “good sleep” for one individual over another for the reasons I previously listed.
There would be little if any practical difference between two 3" support layers and a single 6" support layer if they were all the same type and blend of latex and the same ILD and were inside a tight fitting cover with 3" inches of latex of the same ILD on top of them. Two 3" layers would respond a little more independently and because the elasticity of the top 3" wouldn’t be connected and “pulling back” on the bottom 3" when it compresses and “in theory” it may act a little bit softer but in practical and real life terms most people wouldn’t notice any difference in terms of performance or firmness. If a single 6" core with a 3" comfort layer was a good match for you in terms of PPP then there would be little benefit in having multiple 3" layers that were the equivalent ILD.
Also in “theory only” … two 3" layers that were exactly the same ILD as a single 6" layer could be less durable over the course of a long lifetime because they will act more independently and abrade each other slightly but I don’t think that any difference would be significant or even measurable in “real life” terms and the other factors that affect durability (see post #4 here) such as the firmness of the layers would play a much bigger role. It certainly wouldn’t be a concern of mine.
Multiple layers would have more options for fine tuning though both before and after a purchase because for example a support core with a medium over firm layer would be a little firmer than a medium layer and a little softer than a firm layer (closer to the medium) and you would also have the option to rearrange the layers to firm over medium which would still be in between a medium and firm 6" layer but closer to the firm. You could also use the softer top layer in the middle to create a firmer feel compared to having the soft on top. The main advantage of having more layers in other words is that for those who need it it can provide more options to customize the layer combinations either before a purchase or by rearranging or exchanging layers after a purchase. While this is attractive to some people … it can also add some complexity that may not be necessary or may not justify any extra costs involved in having more layers for others.
Outside of a mattress with more layers having more options to customize the mattress, if a mattress is a good match for you, then one isn’t inherently any better than the other.
[quote]I can find this in a king size is latexmattressfactory.com, and foamorder.com,
they cost 1050$, and 900$ respectively, foamorder seems to have a some what finicky return policy that makes me nervous to order from them, but I also dont want to spend an extra 150 dollars if I dont have to especially with the added cost of the memory foam topper[/quote]
When you’re not able to try a product in person, the best source for information will be a phone call to the manufacturer so that you can speak with a knowledgeable representative who will have your best interests at heart. They will have the best knowledge of their products and how they might compare to others on the market. Additionally, when ordering online, you’ll want to be comfortable with the return/exchange of any online company before you place an order.
Latexmattressfactory.com is a member of this site which means that I think very highly of them and that I believe that they compete well with the best in the industry in terms of their quality, value, service, knowledge, and transparency. I do think highly of foamorder.com in general as well.
It seems that you are attracted to the idea of designing and building your own DIY mattress out of separate components that are purchased from one or several different sources, so the first place I would start is by reading option 3 in post #15 here and the posts it links to (and option #1 and #2 as well) so that you have more realistic expectations and that you are comfortable with the learning curve, uncertainty, trial and error, or in some cases the higher costs that may be involved in the DIY process. While it can certainly be a rewarding project … the best approach to a DIY mattress is a “spirit of adventure” where what you learn and the satisfaction that comes from the process itself is more important than any cost savings you may realize (which may or may not happen).
There is also more about primary or “deep” support and secondary or “surface” support and their relationship to firmness and pressure relief and the “roles” of different layers in a mattress in post #2 here and in post #4 here that may also be helpful in clarifying the difference between “support” and “pressure relief” and “feel” that may be useful as well.
For those who decide to take on the challenge then I would either use the specs (if they are available) of a mattress that you have tested and confirmed is a good match for you in terms of comfort, firmness, and PPP as a reference point or blueprint and try and “match” every layer and component in your reference mattress as closely as possible or alternatively use a “bottom up” approach (see post #2 here).
You certainly are considering the use of some high quality componentry, but it seems that quite a bit of what you are considering is more “in theory” and not much from actual testing of componentry. DIY mattresses can be quite an adventure. 
I look forward to learning about what direction you choose to pursue. I hope amalgamating this information is helpful for you.
Phoenix