Hi NDormio,
Stomach sleeping is the most risky of all the sleeping positions and it’s possible that your mattress may actually be too soft already because stomach sleepers typically need thinner and firmer comfort layers to keep their pelvis from sinking in too far into the mattress and hyperextending their lumbar spine (hammocking). Lower back issues (especially with stomach sleeping) are most often the result of a mattress that is too soft and is allowing you to sink in too far and a topper is usually best used when pressure relief is more of an issue and the mattress is still in good condition. Higher weights also tend to need firmer comfort layers and 3" of very soft foam on top of any mattress that already has soft layers on top could be “risky” for you in terms of alignment. I’m not sure how thick the memory foam layers are in your mattress already but all memory foam is relatively soft and if it’s also fairly thick then you would likely be sinking too far into the mattress over the course of the night even though it may “feel” firm to you if the memory foam is more temperature sensitive and has a slower response.
Other possibilities include a pillow issue because stomach sleepers tend to need a thin pillow to avoid neck and upper body issues but from the sound of things it seems that your main issues are more in the lower back.
It may also be worthwhile trying to use a thin pillow under your upper pelvis/lower abdomen which can help hold up your pelvis a little better.
Since your mattress is about 8 years old it’s also possible that you may have some soft spots in your mattress in which case a topper isn’t the best possible solution because it will just follow and bend into the soft spots in your mattress. If this is an issue then some of the suggestions in post #4 here may be more helpful than a topper.
A down alternative topper has very little support and may provide some “point specific” pressure point relief but but will also reduce the body heat that reaches the memory foam underneath it and reduce its ability to soften and contour to your body shape.
If you are certain that you need a topper for a little extra softness even though it’s not the most likely solution for a back issue … then I would be very cautious about the thickness you use so that you don’t increase the risk of alignment issues with your stomach sleeping. The less the better. Post #8 here has some guidelines that may be helpful in making the best possible topper choice but again I would be very cautious about adding a thick and soft topper on your mattress … especially for a heavier stomach sleeper that is sensitive to hyperextending their spine.
Phoenix