Velocity is defined as the rate of change of displacement with time. It is a vector quantity and has units m/s. The direction of a velocity vector is same as the direction of the motion of body at that instant of time. Magnitude of instantaneous velocity is same as the speed at the instant.
Acceleration is defined as the rate of change of velocity with time. It is a vector quantity. Its unit is m/s2. Constant speed does not guarantee that acceleration is zero. For example a body moving with constant speed in a circle changes its velocity every instant and hence its acceleration is not equal to zero.
Speed is rate of change of distance,it is the measurement of how fast or slow objects move, can be given by formula, Speed=TimeDistance S.I unit of speed is m/s