A particle moving with velocity v is acted by three forces shown by the vector triangle PQR. The velocity of the particle will

As the three forces are represented by three sides of a triangle taken in order, then they will be in equilibrium
$\Rightarrow {\text{F}}_{\text{net}}={\text{F}}_{\text{PQ}}+{\text{F}}_{\text{QR}}+{\text{F}}_{\text{RP}}=0$
${\text{F}}_{\text{net}}=\text{m}\times \text{a}=\text{m}\frac{\text{dv}}{\text{dt}}=0$
$\Rightarrow \frac{\text{dv}}{\text{dt}}=0\text{\hspace{0.17em}\hspace{0.17em}or\hspace{0.17em}\hspace{0.17em}v}=\text{constant}$
So, the velocity of particle remain constant