3000 3-point lift in-op. Help!

Another long shot is if the hitch has draft control. I’ve never been on a tractor with draft control, but conceivably a draft control lever at full light draft could do what you describe. My 1710 has a detend position at the top of the lift lever travel. The detend position is used when an external remote is powered from the 3ph. Unlikely, but conceivably such a feature could be built into draft rather than position control linkages. I hope it’s obvious that I just guessing at some of this stuff, and I don’t know the particular tractor.

A manual would describe linkage adjustments. Linkage adjustments or repairs on most 3ph's can be made without tearing down the hitch. However, you would certainly need a manual if a tear-down is needed.

All sort of things can produce such a problem, and it would be difficult to check without some familiarity with the tractor and a manual. I don't know the tractor myself.

However, a couple of things to check that would not require repairs are: If the tractor has other hydraulics, a divertor valve may be in the auxiliary system position. If a flow control valve is fully closed, the 3ph won't operate, but a pressure relief valve may open and 'squeal' when the lift is raised. My 3ph is plumbed after the loader valves (in an open centred system). The 3ph won't operate if any of the loader valves are operated or stuck.

If there are other hydraulics and they work, then the hydraulic pump is good, and the problem may be the 3ph itself, and except for gross linkage maladjustment, repairs are needed. If no other hydraulics are present, then a pressure test would indicate if the pump is working. A pressure test needs a gauge and knowledge of where to tap into the system.