Assuming that the post about using 3 inch in a 2 3/4 barrel isn't a joke, Aeon's post and illustration is good info. The bottom line for a 2 3/4 shooting a 3 inch is that the 3 inch will fit in the chamber because the shell is closed. Take a fired 2 3/4 shell and measure it. It will measure out at 3 inches. The chamber is big enough for the shell to open up which means the chamber is actually about 3 inches in a 2 3/4 inch barrel. When 3 inch is fired in a 2 3/4 inch barrel the shell opens up to 3 1/4 inches, (and not all the way as it is designed to do), into the forcing cone and, as Aeon's post stated, causes a pressure spike because the shell won't open as it is designed to. You might get away with it for a while but you are running the risk of a catastrophic failure, as Aeon noted, right next to your face.
But, as I always say, pay your money, take your chances.