I'm sure others will chime in too, but I've always been told that 3" tells you you can shoot either 2 3/4" or 3" shells; there's no such thing as a 3" only (to the best of my knowledge). I've shot 2 3/4" shells in guns chambered for 3" shells pretty much all my life.
The one thing you don't ever want to do is try to shove a 3" shell in a 2 3/4" chamber. That never has a good outcome...
The one thing you don't ever want to do is try to shove a 3" shell in a 2 3/4" chamber. That never has a good outcome...