The short answer to your question is that even the smallest 1000w gensets will be able to handle 3 tanks. Of course you didn't tell me how big the heaters are, which really matters.
Honestly, the best course of action is to purchase a Kill-o-Watt (made by P3). They are about $20-30. You plug your stuff into them, and they will tell you how much power is being consumed. I find it indispensable around the house to keep an eye on how much power things are using to make sure I'm not paying too much to run things. With this you can find out exactly how big you need to go.
Also, strongly consider getting a much larger genset. You might as well have enough juice to run your fridge, microwave and any other essentials. Even if you have a gas stove they usually need to be plugged for the oven to work.
You also should consider the kits that have a little transfer panel and put an outlet outside, so you just plug the generator into the house and flip some switches at the breaker panel to transfer critical circuits over. These have the advantage of being able to run your furnace as well, and in NH, I'd expect that to be important. Remember that aquarium heaters are designed to keep an aquarium warm sitting in an already warm room, not a 40 degree room.