Yes
The length of the chamber in a 2 3/4 inch chambered shotgun is 2 3/4". The longer 3" shotgun shells will not fit in the chamber, being 1/4" too long. You can shoot 2 3/4" shotgun shells, of the correct gauge, in a 3" chambered shotgun.
Shotgun Slade - 1959 Madame Vengeance 2-34 was released on: USA: 1961
Shotgun Slade - 1959 Turkey Shoot 2-11 was released on: USA: 26 December 1960
Yes.
Yes you can you can shoot 3 or 2 and a half in
Johnny Ringo - 1959 Shoot the Moon 1-34 was released on: USA: 2 June 1960
Yes, you can.
yes
Factory ammo, yes.
If the gun is marked 2-3/4 on the barrel, that is the maximum length shell that the gun will shoot.
sure - clean it afterward
Yes, shorter shells are no problem.