No matter the barrel length, 3F will give higher velocities than 2F, in a .50 with the same charge weight. Now, will it shoot more accurately with worked up loads?
That is up to the rifle. At any "given" or "equal" velocity, 3F will generate higher pressure which in itself demands tighter ball and patch combinations.
Normally, in order to duplicate accuracy, the velocity must be matched, between 3F loads and 2F loads. Thus, at that load, even though you might have to use 10gr. MORE
2F to get the same velocity and accuracy as 3F, 2Fwill be producing lower pressure.
My short barreled .50 preferred 3F - my long barreled .50 prefers 2F, so far, but I haven't really spent a lot of time shooting it deliberately from the bench - yet. In fact, I've not
tested it at all, with 3F.