The first free body diagram labels all forces acting on the mass
m. There is the force of gravity, that is the weight 
and the tension of the string, that I've labeled 
.
Now something a little weird is necessary. We've drawn a free body diagram for the junction between the three strings. The point is that because the acceleration is zero (and the mass of the junction), the net force acting on the junction must be zero, by the second law. We had to realize that this is important piece of information that we must use to solve the problem.
