RE: can 2 3/4 " slugg fit in 3" chamber
It its simplest terms the "gauge" is the diameter of the inside of the barrel. You can not interchange shells. IE... you can't shoot a 410 in a 20, can't shoot a 20 in a 12. can't shoot a 28 in a 16, etc... etc...
(for all that will correct me I know that the .410 is not a gauge but I am keeping it simple here)
As far as shell length goes you can always shoot shells that are shorter than what it marked on the barrel but not longer than what is marked on the barrel.