To solve this problem, we need to analyze the vector triangle PQR.$\\$The forces acting on the particle are represented by the sides of the triangle PQR.$\\$According to the triangle law of forces, if three forces acting on a particle are$\\$represented in magnitude and direction by the sides of a triangle taken in order,$\\$then these forces are in equilibrium.$\\$In the given triangle PQR, the forces are in equilibrium, meaning the net force$\\$acting on the particle is zero.$\\$Since the net force is zero, there is no acceleration acting on the particle.$\\$According to Newton's first law of motion, if the net external force on a particle$\\$is zero, the particle will continue to move with a constant velocity.$\\$Therefore, the velocity of the particle will remain constant.$\\$This corresponds to Option 3: Remain constant.