Yes
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.
Yes, shorter shells are no problem.
No
yes.
All day long. Any shotgun can handle shorter shells than it is chambered for as long as they are the same gauge.
NO! This is EXTREMELY dangerous!
YES IF ITS MARKED 3 INCH, THEY NEVER MADE BLACK POWDER SHELLS IN 3 INCH FOR 12 GA GUNS.
It will be marked on the barrel.
Depends on the gauge of the shotgun (should be marked on the barrels) and the length of shell it was made for. Some of the earlier guns used shorter shells than are now standard. Just because a shell fits does NOT mean it is safe to fire. If your shotgun is not marked with a shell length (like 2 3/4) you should have it examined by a gunsmith that can check shell length.
You could eaither shoot dove with a 20 gauge shotgun or a 12 gauge shotgun. You should also use a bird shot shells
yes
No, you can not shoot anything but 12 gauge shotgun shells through a 12 gauge shotgun. Never use any ammunition other than what the firearm was designed for.