Absolutely you can use it. I have used it in .40, 45, .50, .54 and .58 caliber rifles, including flintlocks in all but the .45 caliber. I have used Pyrodex P (equal to 3F) and Pyrodex RS (like 2F black) and Hodgdon Triple 7 loose powder in 2F and 3F grade. You need a black powder booster charge of about 5 grains, which it says right on the Triple 7 bottles, and you need real black powder in the pan. Then a flintlock will fire reliably. The Triple 7 powder burns hotter than real black powder, so you may need a thicker patch and/or an over-powder wad, which can be just a wadded up regular shooting patch. I have shot hundreds of rounds of these powders through my flintlocks and they are totally reliable.
What I do is fill my powder measure with the charge of Pyrodex or Triple 7, then tap it to settle it about 5 grains, and top that off with real black powder, 4F, 3F, or 2F, makes no difference, dump it down the bore, seat the ball and then prime the pan with real black powder, and 4F, 3F or 2F each will work well. It is absolutely reliable in a flintlock when loaded in this manner.