Let's call a the acceleration of the system. The problem says that the block m3 is static, so the acceleration is zero: a=0.
Calling
the tension of the string between m1 and m3, and 
the tension of the string between m2 and m3, the problem can be solved by writing the following system of equations:
However, we know that a=0 and the problem asks only for, so we just need to solve the first equation:
and so
Calling
However, we know that a=0 and the problem asks only for
and so