My knowledge of hydraulics is very limited but here's a couple thoughts.
Has there been any changes or removal/reinstall of anything on the tractor lately? E.g. any hydraulic lines or loader removal? Wondering if any hyd lines got crossed or one is not fully engaged.
Do you have a "power beyond" kit installed?
If you have a loader, try operating the loader and see if anything happens with the 3PH.
Also check Youtube videos. Search for something like "3PH won't move".
