what is the principle of superposition of electrostatic force

 Priciple of Superposition of Electrostatic forces states that the NET force acting on a given charge due to the charges present in its surroundings is the VECTOR SUM of the forces acting on that given charge due to those all charges present in its surroundings.

It means, you should find out the forces acting on a given charge by each charge near it (using Coulomb's law) & then add all of them using Vector addition.

Superposition principle seems to be a normal vector addition law, but it has a hidden meaning in it i.e. the force acted by a charge on a given charge is unaffected by presence of any other charges around it. That means, the force is still calculated by Coulomb's law and there is no new formula for it when other charges are present in their vicinity.

Principle Of Superimposition For Electrostatic Forces

In A System If There Are More Than Two Charges Then Net Force On Any Charge Can Be Given By Vector Sum Of All Individual Forces Acting On It By Independent Charge Of The System.

F_net {Vector}= F₁+F₂+ F₃+F₄ +F₅+ ----------- + F_n