I haven't had any problems with 3 x 24 tires on 24 inch mountain bike rims, so I would think you could do the same with 26ers. The problems will probably be the frame, chain and sprocket ares.
Kenda Flame on a drop center = 2-3/4" rim is about 1" wide where bead seats
Kenda Kraze on a ballooner rim = 2-7/8" rim is about 1-1/4" wide where bead seats