You know that work is the variation of mechanical energy. Consider the amount of water that flows in one second, i.e., 3kg. What's the initial mechanical energy of this amount of water? And its final mechanical energy when it reaches the top (remember to consider both gravitational potential energy and kinetic energy)? This should allow you to find the work done by the pump in one second, which is the pump's power (work over time is the definition of power).