The FF rating deals with the size of the grains of powder.

1 F = cannon powder
2 F = rifle & shotgun (normally considered .50 caliber and larger)
3 F = pistol & small caliber rifles (.45 caliber and under)
4 F = priming powder

You can use 3f powder in rifles .50 caliber and larger. In fact some rifles do much better with it. But only your rifle will tell you whether it likes 3F powder better then 2F. If your rifle shoots well with 2F then I would stick with it. Although it is fun to experiment. I shoot 3F powder in a lot of my larger bore rifles with excellent results.