At a certain moment of time, velocities of 1 and 2 both are 1 m/s upwards. Find the velocity of 3 at that moment.

We knowV2 + V32  = VP  ∴   1+V32  =−1  ⇒  V3=−3m/s(- sign denotes that the block is moving in downward direction with velocity 3m/s)