Is it a bad idea to do research with multiple different professors at the same time?
There comes a point where a person will be spread too thin – that's inevitable. Only you can really say how much you can take on before your workload starts to interfere with your academic goals and achievements. 

If you're already doing the work of two students, I'd be cautious about expanding that to three. 


  I'm spending around 9 hours a day on research right now with ~3 hours in class, 1 hour eating, and 4 hours free time, on week days, and even more free time on weekends


You make it sound like those four hours of free time are being wasted, when they might not be. Perhaps your brain is appreciating that rest more than you know. And I don't see any time allotted for physical fitness; besides being good for your health, fitness can be good for your mind. 

If you decide you want to try a third project, you should be honest with yourself, and up front with the sponsoring faculty member from the outset. "I'm already working on two other projects, so, to be honest, this might turn out to be too much for me. But I'd still like to give it a try." Such a disclosure might make it a little harder for you to find someone willing to take you on, but it might help save your reputation if you find yourself in a position where you've bitten off more than you can chew.