Be careful about plugging your lights into a generator. Many generators make fairly dirty power and aren't suitable for powering studio strobes. You will see inconsistent behavior from your lights unless you have a way to smooth out the power. There are generators that clean up the power they deliver, and those will work fine. If you want to shoot all day, a decent generator will probably be the least expensive way to solve your problem. You can't really run them indoors in a closed space though without killing yourself (Carbon monoxide).
A powerful UPS (Uninterruptable Power Supply, like those used with computers) that delivers a pure sine wave output works very nicely. But if you want to power all three of your lights at full power, you're going to need a 5000VA or better UPS, due to the current draw of your lights (~35 amps). That is going to get expensive real quick. Nevermind that a UPS like that will weigh ~300 pounds.
Dyna-Lite makes the XP1100 inverter. You could use one of those to power your (2) 600J units and another to power your 1200J unit, as long as you knocked the power down on everything by 1/3 stop. Two of those cost the same as the 5000VA UPS mentioned above, but they only weigh 22 pounds each. Additional batteries are relatively cheap and easily replacable.
People who do stupid things with dangerous substances often die! -me