Hi phewd,
3 lb memory foam will soften “more” (not less) and more rapidly over time than higher density 4 and 5 lb memory foams and as you can see in the guidelines here … I would be very cautious about a mattress that has more than “about an inch or so” of lower quality/density memory foam (less than 4 lb density) in the upper layers of a mattress because once you reach 2" or more of lower quality/density memory foam in the upper layers of a mattress it would have a much greater chance of being a weak link in the mattress in terms of durability because of premature foam softening and breakdown and I would be very cautious about considering it unless there are no other reasonable alternatives in its budget range that use higher quality/density and more durable materials (memory foam or otherwise).
I would normally use 4 lb as a minimum density guideline for memory foam (which would be more durable than 3 lb memory foam) but in higher weight ranges I would tend to focus more on 5 lb memory foam and reduce or minimize the use of 4 lb memory foam as well because higher density 5 lb memory foams would be more durable than 4 lb memory foams and a density that would be suitable for lower weight ranges may not hold up as well if you are in a higher weight range.
I would also keep in mind that the loss of comfort and support that comes from foam softening isn’t considered to be a manufacturing defect and isn’t covered by a warranty unless there is also a visible impression in the mattress that is deeper than the warranty exclusion which is less common (see post #174 here) and the loss of comfort and support is the main reason that you will need to replace a mattress.
Phoenix