A particle moving with velocity $\vec{V}$ is acted by three forces shown by the vector triangle $PQR$. The velocity of the particle will:

The vector triangle $PQR$ shows three forces ${\vec{F}}_1,{\vec{F}}_2,{\vec{F}}_3$ acting in a closed loop (cyclic order).In vector addition, if vectors form a closed polygon, their resultant is zero.
$${\vec{F}}_{net}={\vec{F}}_1+{\vec{F}}_2+{\vec{F}}_3=0$$
According to Newton's First Law, if the net force is zero, there is no acceleration.Therefore, the velocity of the particle remains constant.