Tony G,
I am still not sure how you did the test.
Could you do it again like this.
Plug your gage into the work ports hose, closest to the valve lever .
Have the engine running at about 1200 rpm.
Pull the lever so part of the spool shaft comes out of the valve, momentarily, and read the pressure.
Did the pressure relief activate, and did the engine slow down?
Run the engine to 2500 rpm and do the same thing. Read the pressure.
On the 3pt, if you can raise the bale and then release the lever, the spool should block any fluid flow in neutral if things are good.
If the bale descends, then you have bad 3pt cyl seals or a worn spool in the 3pt valve.
For permanent installation of the pressure gage, install a tee at the valve IN port and use fittings to install the 3000 psi hyd gage.
Anything you do using hyd will be reflected on the gage, be it loader operation or 3pt operation. Use it to monitor your hyd system and for trouble shooting .