drilling out a GP is very very risky. You could wind up leaving bits of metal in the combust chamber, then you will have problems when you go to crank the engine.
Or you could damage the threads in the aluminum head then the new GP won't go in without chasing threads, again some risk.
With three busted GP's I would plan on sending the head to a reputable shop, one that has experience in this. I saw a machine shop where they were removing the remnants of a GP in a 606 head. The MB Dealer sends the heads there when they bung them up
The proper way to machine a GP out is to set the head perfectly flat in a fixture with an overhead mill and step machine it out using reamers until most of the metal is gone then chase the threads out with a special tap (by hand!)
I would be too nervous to try tapping out one plug with the head on the engine much less three, just thinking about the price for a replacement head I might go nuts worrying and clam up $$$$$$$$$