Hi Solstice,
Welcome to the Mattress Forum! 
My concern with the configuration you’ve described would be the middle layer of the 3 lb memory foam, which would be a lower durability than I would recommend. You can read more about the durability guidelines here.
While you may have already read it, regarding a DIY mattress I would start 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).
The items you’ve put together certainly could create a mattress that many might consider “comfortable”, but unless you’ve already tested out something in person using similar componentry then the only real way to tell how it would feel would be through your own personal testing once you had all of the layers assembled.
Phoenix