My money is on Legends simply not getting an episode that week, but the Arrow-verse has surprised me before.
It’s possible that The CW will simply skip an episode of Legends that week and perhaps incorporate some of those heroes into the action without dedicating a whole fourth hour to the crossover.
We can’t say at this point if Legends of Tomorrow will air a solo episode in its regular Monday time slot, which would place it behind the Arrow leg of the crossover, assuming Arrow keeps its 8 p.m. slot on that Monday.