A particle moving with velocity $\overrightarrow{\mathrm{v}}$ is acted by three forces shown by the vector triangle PQR. The velocity of the particle will

Given vector triangle.
As the three forces acting on the particle are represented by three sides of a triangle, taken in order net force acting must be zero (Triangle law of forces). So, velocity remains constant as net force is zero.

$\begin{array}{rl} & {\overrightarrow{\mathrm{F}}}_{\text{net}}=0\\ & \Rightarrow \mathrm{m}\frac{\mathrm{d}\overrightarrow{\mathrm{v}}}{\mathrm{dt}}=0\end{array}$

$\frac{\mathrm{dv}}{\mathrm{dt}}=0 \text{means }\mathrm{v} \text{is constant.}$