We observe from the given displacement time graph above, that the slope is a straight line inclined with time axis, so the body is moving with uniform velocity. Hence, the velocity will be same at t = 1 s, 2 s and 3 s.
Velocity = Slope of straight line OP
= AB\over OB
= 2 - 0\over 1 - 0
= 2 m s-1
Hence, velocity at t = 1 s, 2 s and 3 s is equal to 2 m s-1
Velocity-time graph for the motion of a body is given below: