Mesh analysis with 3 loops
First hint refer to diagram below, can you find the voltage drop across R3. If the values of all R and V is given. 



Edit :
Second hint refer to your original diagram, (I3 - I1)(30) + I3(20) + (I3 - I2)(40) = 0. Refer to diagram below for the direction of the current assumed. Try browsing through this link for better understandings.


Example:

Refer to the picture below if you should get the correct equation by following the steps in the image Image Source

I feel that you are asking a fundamental question in electrical engineering, that's only reason for me not to give you a direct answer or solution instead I'll try my best to guide you to get the solution.