The hydraulic pump power (in HP or Watts) is Q*P, where Q is the flow rate at the pressure P: power=flow*pressure.
The pressure is actually pressure differential. For a 3000 psig pump, just use 3000 psi. The engine (or motor) needs to output the required horsepower at the rated RPM of the pump. As such, a gearbox may be needed  dependent on the pump.
As an example: a 1 gpm hydraulic pump operating at 3000 psi needs 1.75 horsepower (hp) or ~ 1.3 kW of power.
power = (1 gal/min)*(231 in^3/gal)*(3000lb/in^2)*(1min)/(60sec)*(1 ft/12 in)*(1 hp/550 ftlb/s)= 1.75 hp
Hydraulic pumps that produce 3000 psi are positive displacement pumps  which means flow = displacement/revolution * RPM. You can have a more powerful engine than required. A normally closed directional valve can be used, but cooling for the full pump horsepower may be needed  such as a reservoir heat exchanger, dependent upon operation. Good luck!
