Yeah, F$ is right. You could do it if the 850W is a strong one, and you weren't worrying about the longevity of it...kind of like just "running it till it dies" mode.
If you have plans for the future of that 850W, better to throw it in another rig with only 2 GPUs, and get the 1KW PSU for the 3 GPU rig. That's why I got the 1KW PSU too...someday I will be able to fill that 3rd slot on the DrPop rig!