The physics
Superposition: each charge acts on the test charge on its own. The other charges do not change that force. The net force is the sum of all these forces, added as arrows.
F = k q₁ q₂ / r² (k = 9×10⁹ N m²/C²)
Fnet = F₁ + F₂ + F₃ + … (add head to tail)
Like charges repel: the force points away from the source. Unlike charges attract: it points toward the source. Gravity always attracts: F = G m₁ m₂ / r².
Two forces at right angles: |F| = √(F₁² + F₂²). Two equal forces with angle θ between them: |F| = 2F cos(θ/2).