How to calculate force between multiple charges. .

Force due to one charge is independent of others. We can use superposition principle.

apply coulombs law for each charge w.r.t to other and add as vectors. decide direction suitably

Vector addition

As the force of colomb is free from the influence of pesence of other charges you just calculate the force on first charge due to others, force on second charge due to others(except first) and so on, then add them using superpostion principle you will get our answer.

ALL THE BEST.