Honestly, if you don't treat that high-cap like a water-hose, i wouldn't worry too much about it. I think with that M3, you are limited to the mags you can get for it, correct? It's not like it's an M4/M16 where every manufacturer out there supplies a variety of mags in real cap 30's, to mid cap 68's/80's/120's etc..
So, if you real-cap your high-caps, then to run any skirmishes, you are likely going to need to carry, what 10+ mags? Seems like a waste of money to me, if the objective is realism.. considering that those 10 "real-caps" are in actual fact now tactical morocca's announcing your presence to everyone within 500 yards of you..
I'd just stick to a couple high-caps (if that is all that is available) filled up, and apply a disciplined trigger finger with them.. my $.02 anyway.