It it is so marked and the owner's manual says so.
yes it will. the rating shotguns are chambered for is the maximum size shell you can shoot. i have a 1100 LT chambered for 3 inch and i shoot 2 3/4 inch shells out of it mostly.
sure - clean it afterward
Yes, shorter shells are no problem.
In general, no. Unless the gun is marked for 3 inch (or even 3 1/2 inch) shells, you should not attempt to fire 3 inch Magnum shells, even if they seem to fit. However, if you have a gun made for 2 3/4 inch shells, and have 2 3/4 inch "magnum" shells, IF your gun is safe to shoot, it should be safe to shoot those "baby" magnums.
YES IF ITS MARKED 3 INCH, THEY NEVER MADE BLACK POWDER SHELLS IN 3 INCH FOR 12 GA GUNS.
Your question needs some work- as written, has no meaning. A shotgun marked 3 inch can shoot 2 3/4 inch shells, or 3 inch. A gun marked 2 3/4 inch can only shoot those- no 3 inch shells.
You can fire 2 and 3/4 inch shells in a shotgun chambered for 3 inch shells. There is no 3 and 3/4 inch shell made.
Yes
No made for 2and a half inch shells
yes
Yes, you can shoot 2 1/2,2 3/4, and 3 inch shot gun shell in that barrel.