Hi Kristy93292.
Like all other components in a mattress, latex too comes in higher ILDs that would make it an ideal supportive layer within a mattress (Firm, Xfirm between 32 to 44 ILD would be a good application of it). The goal is to determine the best layering configuration for you … but from the description of your current setup, you are clearly missing the support (base layer) that would ensure that you have enough primary support to keep your spine in neutral alignment, the 3" of medium Dunlop would help with some of the secondary support needed but it would not be enough to provide adequate primary support. 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 as you go through this process.
I was not able to find your previous posting detailing this but generally, I’d keep in mind that the different layers in a mattress are usually designed in such a way that the complete mattress will have both supportive qualities and pressure relieving qualities. The core of the mattress which is the middle and bottom parts (usually innersprings, latex, or higher quality polyfoam) is the part that is primarily responsible for supporting the heavier parts of your body and keeping them from sinking in too far.
I can certainly understand your frustration … not having a supportive sleeping surface is usually the main reason for lower back pains and this is also the main difference between sinking in and sinking down I’ve been using in the hammock example. If you lie on a hammock your middle parts will sink “DOWN” further than the upper and lower parts of your body even though you are not sinking “IN” to the surface of the hammock at all. Sinking "IN is all about the depth or your pressure relieving cradle while sinking DOWN is all about whether one part is “traveling” too far relative to the others.
The first step is to try to work out a way to determine if the heavier parts of your body are sinking in too far and add an appropriate base layer. You did not mention your BMI and sleeping position which also are important in assuring that yoiu do not have too thick/soft comfort layer for your needs. Ultimately only you can feel what you feel on the mattress because we are all built differently and have different needs and preferences which makes it a bit of a trial and error process. This is all part of the 'art and science" of mattress design and is part of the reason why working with an “expert” can be so valuable. Your “job” is to describe the symptoms as accurately and specifically as you can … their role is to use their knowledge and experience to the best of their ability to help you decide on the types of changes that have the best odds of solving the issues you are facing with your DIY project.
Phoenix