Assuming there's nothing mechanically busted internally which is unlikely, you have two possible valves. The three-point works as a hydraulic ram so you've got a piston and a cylinder. You've got a valve to pump fluid into your cylinder which is controlled by your three-point lever and then you've got a second valve to release fluid out of the cylinder which is controlled by your 3-point leak down. So either you've got one valve stuck open or the other one stuck shut.
Release valve controlled by your leak down knob is usually considerably simpler to access and take apart so I would start there first. Take it apart and see if anything's obviously broken or stuck