Welcome to the forum.
Not sure what tractor you are talking about, but some require that the transmission be running (clutch out and either in gear moving, or in neutral, not moving) so the PTO is operating (hyd. pump operating) while using the 3pt arms. Hopefully this makes sense, and solves your problem.
Otherwise, there may be a knob that opens and closes the hyd to the 3pt arms.
More bio info and tractor info would be helpful, to get to the crux of your questions.