RE: can 2 3/4 " slugg fit in 3" chamber
Actually, the 20 is a gauge and the .410 is a caliber. You cannot shoot 20 ga in a 410 nor a 410 in a 20 ga if only because of the obvious size difference.
In your 410 single shot, as long as it has 3" marked on the barrel, you can shoot 2 1/2", 2 3/4" or 3" 410 shells in it without any fear of damage.