If I understand well how you draw the diagram then
where c is one side of the square.
The forces are balanced.
In the direction of Q
I think you should get
The electric force between two particles is proportional to each of the charges and inversely proportional to the square of the distance between them
The force of q1 on q2 is the same as q2 on q1. This is action-reaction principle.
Think of it as gravitation. It is the same. Mass is gravitational charge and "charge" is electric charge. The force of the moon on the earth is the same as the earth on the moon. Effects are different because mass of each is different.
So the force of Q on Q is and the force of q on q is Notice that when the two charge are the same sign they tend to repulse each other. The opposite happen when they are of different sign. Here, Q and Q are obviously the same sign. Two charge q have to compensate for one charge Q that is farther.