That prob depends more on what you're trying to crank that aynthing. The 3S will run it, just won't have the rpm a 4S would give it. If its a new, tight pinch engine, prob not going to start it. But if its an older engine with some time on it, might be able to. Only way to really find out is to try.

Hook up batts in parallel so the voltage will stay same and get the increased runtime.