How to find tension on pulley with 3 masses
 

Hello!

I'm having a dilemma. I know how to find tension (T=Mmg/M+m) but this is with one mass being pulled by another.

http://www.webassign.net/hrw/W0084-N.jpg

From this picture, and a few masses I'm given, I have to find the tension of the rope between B-C. What am I missing?

I'm not sure what your formula is supposed to represent but I do know that you can simply solve this problem using three free body diagrams.
If this still doesn't help you, come back and we'll give you more detailed help but, really, you should be able to take it from here...

If it helps you, for your first FBD (Free Body Diagram) simply combine weights B and C. The forces between the two are unimportant at this time. After you have your solution to that, you can break it back out into B and C and should be able to solve your problem.