Not an Avionics tech, but I have seen something similar. From memory #3 Gen powers the Main AC Bus, but when #2 tried to power the Ess AC Bus there was an issue that would cause the three Gens to individually fall as each tried to power the Ess Bus. So the thinking was a loose connection or faulty earth. Now to throw a spanner in the works, in this case it ended up being an issue with one of the Props (Can't remember which one) De-Ice Timer Units being dodgy that created a similar effect.
Good luck.
UPDATE: Spoke to the AvTECH on the job, the issue ended up being Deicing Eng 3 short.
