RE: rem 1100 3"??
NO. The reciever on 2 3/4' and 3' mod 1100 are different lengths, the barrels are not interchangeable. To shoot 3" you have to have a 3" reciever and a 3" chamber in the barrel. The newer 1187 has interchangable barrels for 2 3/4 and 3". As far as your friend accidentaly shooting a 3" in a 2 3/4 " gun , that is a serious no no. The gun could blow up in his face. Don't do it!
One option that you might try is to take your 2 3/4 " barrel to a gunsmith and have the chamber cut to 3". this would solve any high pressure issues with a 3" shell, but it might not feed or eject correctly due to the extra length of the shell. I did this with my B-2000 it was cheaper than getting a new barrel.It works great.