Hi Cruise,
As you can see in the guidelines here … the top 3" layer of memory foam is a little bit lower density (under 4 lbs) than I would be comfortable with in this price range and would be somewhat of a weak link in the mattress. The middle and base layer use good quality materials (2 lb polyfoam) and while the convoluting in the middle layer would reduce its durability … this would be less critical with 3" of memory foam above it and also with a better quality 2 lb polyfoam material.
The tutorial post includes a link to this list of some of the better quality/value online memory foam options I’m aware of and post #4 here includes some of the lower budget options I’m aware of that use higher quality memory foam in the top layers which are the weakest link of a mattress.
There is more about the most important parts of the value of a mattress purchase in post #13 here that can help you make more meaningful comparisons between mattresses. Costco won’t be able to offer any good guidance about the suitability of a mattress for particular body types or sleeping positions (they are not particularly knowledgeable about mattresses) but they do have a great return policy so you would be getting a “free roll of the dice” and if the mattress doesn’t turn out to be as suitable as you hoped for then you can return it which reduces the risk of making an unsuitable choice with an online purchase.
Overall … I normally wouldn’t suggest considering a memory foam mattress that has a density in the lower 3 lb range and while it would be a “better than average” choice compared to many other mainstream mattresses, it certainly wouldn’t be my first choice considering the other options that are available to you.
Phoenix