The Three Doctors - The Time Lords come under threat from a power originating from a black hole which drains away the power from Gallifrey. The Time Lords decided to break their own laws of time and reunite the three doctors.
The Five Doctors - Each Doctor is taken out of their respective time streams to play a deadly game (The Game of Rassilon) on Gallifrey.
The Two Doctors - One of the Doctors (Patrick Troughton) is captured so that the Sontarans can extract some DNA for their time travel plans. This affects the current Doctor (Colin Baker) and he decides to go visit Dastari, the only person he knows that can understand what's happening to him. Dastari is in cahoots with the Sontarans and thus the Doctor meets with a previous incarnation.
So in two storylines the Doctors are taken out of their time streams, and in another their paths just cross. Won't be too difficult to develop a storyline for them.
What would be good is if one of the Doctors could now be a renegade

Or even more convoluted, the Doctor is the Master