Physics help with a triangle?!?

Since the force on a charge is equal to the electric field acting on the charge times the magnitude of the charge, you should first find the net electric field on one of the charges. Let's evaluate the force on Q1. What you would do is find the electric field of Q2 at Q1 and add that to the electric field of Q3 at Q1. Since the charges have the same magnitude, and are equidistant from Q1, the x-components should cancel out and the y-components should be added together.