From what I understand and have read many times on these forums is if you are using 2F and want to switch to 3F drop the grains by 15% to 20%.

I recently switched from using 100 grains of Pyrodex RS (2F) to 85 grains of Pyrodex P (3F). It seems like it is more accurate and a bit "cleaner". Clean is a relative term in black powder shooting.

Hope this helps.