It's perfectly safe and acceptable to fire 2 3/4" shells in a Super Magnum 3 1/2" chambered gun. The patterns will be fine too.
If it's a semi-auto gun, the gun may not cycle the next round into the chamber due to the low recoil/gas of the shorter round, but still it's safe to shoot the shorter shells in the longer chamber....... assuming the shells are the same gauge of course.