Since weather like wind and light conditions as well as the type rest and the capabilities of the shooter all enter into ityour 3/3 can not really apply.
A lot of us use and talk about 3 shot groups they are the most commen when narrowing options to fine the best load.
Weather it is a good group or not may depend on iron sights or not, the type of gun Hawken ,inline or kentucky rifle. I consider 3 inches a good group for my iron sighted Hawken with PRB,its awful for my Omega with one of the load that will shoot under 1.5 inches with 5 shots.
Much of this is relative to the context it is in. Lee