I've been debating several times whether or not alternate futures can affect the past. Ultimately I came to the conclusion they can;
In the trip back to 1955 in part II we see Marty from part I (twin pines mall) as well as biff from 2015 lone pines mall timeline despite the fact that both these timelines were not current one and biff may be dead by 2015 in that timeline.
In episodes 3 and 4 of the game which were FCB timeline, there is plenty of evidence of 'old doc' still making the trip to 1931 (him being in FCB's picture, the fact everyone still remembers Carl Sagan)
So I'm sure we are in agreement that 2015 will not play out the same way we saw in part II due to changes to the timeline. So lets say in the current version of the timeline, biff gets a time machine again in 2015 and travels back to november 12th 1955, we'd have 2 different versions of Biff from 2 timelines. So this is the same rationale you can use for the 3 marty's.