Newton's third law: When two particles interact, the force F which the first exerts on the second, is equal and opposite to the force F that the second exerts on the first, both being on the straight line that joins the particles. The process is simultaneous.
The following image represents the interaction of two bodies, 1 and 2 which repel each other.
We write F_1/2 to indicate the force that body 1 exerts on body 2 and F _2/1 to indicate the force which body 2 exerts on 1. They are equal and opposite. They are applied on different bodies (if they applied to the same body they would cancel each other out).

The forces exist while the interaction lasts. You can revise interactions here.
The following visual represents the interaction between two skaters. It represents the corresponding pair of forces.