I had MGIIIa also and only replaced one tweeter in 7 years, and that was because of a movie soundtrack clipping my 255watt a channel amplifier. Movies would melt the fuse adhesive and the fuse would appear good, but wasn't. Make sure the fuses you are using are fast blow, not slow blow and the correct size.
Also are you bi-amping the 3.5's without the external crossover? This can cause problems.
Low line voltage might be starving your amplifier when it craves the current.
My guess is you are popping tweaters because you don't have enough current. Think 4ohm and get as much power as you can afford. I used my ARC D400MkII to moderately loud without any issues and my EAD PowerMaster 2000 (800watts into 4ohm) was even better.
Look at Bryston, EAD, or some other high current 4ohm stable load amplifier and check those fuses.