It used to be that if you poured lots of ammo into the cockpit of the lead buff and blew it up, the other two would pop as well. Someone once told me that this was because the buff pilot had been killed.
I haven't had this happen in months and months, it seems. As for three in one pass, I have managed it in a Chog, Tempest, A8 and P47 over the last few months, always from the side and always pouring fire into the first drone (sort of hiding behind it so the pilot in the main buff won't get a bead on me), then the main buff , then the second drone. Risky business, but it works every now and then.