Three drones? That'll be one tough son-of-a-gun!
I've never tried that. Are you certain that all three drones aren't inside each other? Can one shot from a single beam weapon take it out or not?
If necessary, just make three ships attached to each other. Write a mission script that has one drone-equipped core ship moving after the player (push radius zero) and two other drone-equipped ships that are constantly moved to just a few meters away from the core ship.
"Damn the torpedoes! Four bells, Captain Drayton!"
(Likely actual words of Admiral David Farragut, USN, at the battle of Mobile Bay. Four bells was the signal for the engine room to make full steam ahead).