This isn't an NFA question, but I'm sure the mods will be along shortly to move this thread to the shotgun forum.
While we're here though, we may as well address it. The H&R Pardner shotgun is a single shot break-action shotgun. If your example says that it's chambered for 3" shells, it's perfectly safe to shoot 2 3/4 shells in it. Shooting a 3.5 inch shell would cause an increase in pressures that wouldn't be good for the gun, nor for your shoulder.
I've fired 3.5 inch 10 gauge shells from a properly chambered H&R shotgun and the recoil was memorable.
Short answer: Yes, you can shoot either 2 3/4 or 3" shells from that shotgun.