No, you will not feel the "gaps" in a tri-fold mattress, for the reason Maki said. The quality and attributes of foam in them varies, and I agree that you will need to replace the foam fairly often. The tri-fold format has a lot of advantages, IMO. At one time, we tried going with a higher density foam for longevity, but found it too heavy to fold and unfold every day after awhile.
There is a lot to discover about foam thickness, density and firmness and how those things inter-relate. More than one type of foam may be what you ultimately find most comfortable, but would also recommend keeping it relatively inexpensive at the outset. I agree that the guys at Military Supply House really know their stuff!
Our bed was super comfortable, until it wasn''t (again). Last week, we replaced the core foam with a 3" thick piece of HQ HD36 from Foam Factory. They call it "medium" firmness, but I have used their foams before and find both varieties of the HD36 quite firm. Adding additional thickness to it makes it downright hard IMO (yes, I've tried that). The HQ (2.5 density) holds up much better to our use (300#'s combined; active).
This time around, I've added a 2" thick "soft" latex topper (we hate memory foam). Hospitology makes an awesome, washable "down alternative" mattress pad that we also use.
I'll post an update after a couple nights' sleep.