When designing hydraulically-driven machines you have to match the power able to be transmitted through the oil to the power at the engine and the power you want available at the motor shaft. The power in the oil can be calculated with the equation GPM * PSI / 1714 assuming perfect efficiency. (Hydraulic drives are anything but efficient but this makes the math easier and it will be in the ballpark.) Using the 3 gpm you said your pump puts out gives only about 5 horsepower at 3000 psi that can be transmitted through the hydraulics. It makes no difference what kind of motor you have on the other end, you simply won't get more than 5 hp out of it at this pressure. To transmit the full engine power at 3000 psi will require a pump that can put out more like 18 gpm. Next, use the equation DISPLACEMENT (in^3) = GPM × 231 / RPM to size the motor to run at the speed you want. You can also use the equation TORQUE (in-lb) = DISPLACEMENT (in^3) * PSI / 6.28 to determine the motor's torque output. Assuming you aren't hydraulic power limited the torque output from a motor is dependent entirely on displacement and pressure - the rotational speed doesn't factor in.
As Bob noted, you'll need some way to vary the flow rate to the motor. A flow control valve can be used but this would be an extremely inefficient way of doing it as the entire pump flow rate will be raised to whatever pressure the motor requires to turn, even if the valve is sending only a small percentage of the pump's flow to the motor. The rest of the engine's power will be converted into heat through the valve. A better way would be to use a hydrostatic pump and motor (also like Bob mentioned) where the pump's flow rate can be varied from zero to the maximum. However, when you get into hydrostatic drives you also need to provide filtered charge oil and a variety of other considerations. This would be a difficult project to get working right the first time for a person without a background in such things and the trial-and-error method could be prohibitively expensive given the cost of hydraulic components.
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