Tap the heads of the bolts with a hammer repeatedly along with the methods the others describe.
The middle alternator bolt has been missing {broken off} for God knows how long on my '99. No ill effects at all, the top & bottom bolts are holding it just fine. Not the end of the world if the middle bolt breaks.

I stuck a chrome plug over the hole, I'll deal with it when I need to pull the alternator in the future.

Steel bolt in an aluminum case can be a PITA!