If the battery even after a charge wont start the car and goes down to 11 volts its got a bad cell. Ive seen it happen on redtops before shawn. My buddy put a brand new redtop in his 240 and less than 6 months later it had a bad cell only getting 10.5-11 volts from it right after a charge.
Your car may still have a drain causing the battery to go bad as did my buddies car. Ended up being his trunk light was staying on because the switch on the latch was messed up. Popped the bulb out and no more drain.
Like Ben said, use a multimeter between the positive terminal and the positive cable disconnected from the battery and see if you have an amperage reading. You should see < .10 amps with the key off. On a good stock electrically sound car I usually see about .03 amp draw car off. Basicly thats just your ecu's backup power and CD player internal clock drawing that small amount. You will notice it if anything is out of the ordinary. My buddies with just his trunk light staying on it was drawing 1.1 amps, Removed the bulb and whalla, down to .03 amps.
But im gonna vote your battery is bad as the main issue but the cause for killing 3 batteries quickly there is most likely a draw somewhere. Unless its just some bad luck. lol