Is it a bad idea to do research with multiple different professors at the same time? I want to go to graduate school to study computational molecular biology and biochemistry. 

During my first three semesters as an undergraduate I finished my distribution requirements and started doing research with one professor.  During my fourth semester I started doing research with two more professors (working in computational biophysics and  computational linear algebra).  My work with two of these three professors has already led to work that will be published soon.  I am considering starting research with yet another professor (in statistical genomics).

Is it wise to begin research with yet another professor?  I fear that having engaged with too many professors makes my resume read like that of a dilettante rather than a promising computational biologist.
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.