If you need to fit triple bunks, they're going to be pretty short. No way around it unless you opt for a massive roof raise.
You can gain a little bit of space back by using a thinner mattress. I am a firm believer (pun intended) that mattress thickness does not correlate to mattress comfort. We have a fleet of Paco Pads, which are a simple 2" chunk of firm foam wrapped in tough PVC shell. I flop those things directly onto roots, pinecones, cobblestone beaches, etc, and sleep better than in my bed at home. I'd say get thin but firm standard foam to start. The thinner you go, the firmer the mattress has to be. I like the "LUX" from Foam Factory (
foambymail.com). If you occupants complain, you can always add extra thickness or a memory foam topper later.
My second suggestion is to not add permanent aisle walls in a bunk that short. We recently finished our bunks (27.5" wide, 78" long, 27" clearance above 2" mattress). They have a 1/3 permanent wall, and it is surprising how much that closes everything in and complicates entry and exit. I can still get in there as a 6'2", 200# guy, but any more wall would have made it very difficult indeed. With a shorter bunk, I think a wall would be even more of a pain.
Good luck!