if you select training PF/C the number of players who play on this positions is not relevant
if there's only 1 PF and 1 C the training speed will be the same for 3 PF and 3 C (asuming each plays 48 mins/week)
but changes if you choose to train SF/PF/C or C and again with no matter how many players on each position...
Howl to the Moon, you won't regret it.