Talking strictly pre-snap, F and S always have #1 to their side.
Then it becomes an IF/then decision.
If # of backs = 0
Then L and H have #2 on their sides and B has #3 wherever he is
If # of backs = 1
If no trips or trips to H
Then L has the back and H has #2 on his side and B has the remaining eligible
If trips to L
Then L has #2 on his side and H has the back, B has #3 to L
If # of backs >=2
Then for the most part you let them declare as they come out of the backfield, however typically if there are two L and H take the back to their side.
As far as I am aware on or off doesn't matter unless they are stacked.