3” is what I have here. Once opened it get flat immediately, it’s exactly what I wanted. I have pain in my low back after babies. So I needed something. I have this on top of a plush mattress and it is perfect. It came like a roll. It doesn’t move and  it is a silky type material that does create some heat. It is a very easy fix, just take it off if you don’t like it. Overall, I am satisfied with my purchase.