The 3PT is the last device in the flow path.
Shows what I know!
I remembered reading the 3PH had priority, so imagined if it were activated everything downstream would only get partial flow until the 3PH lever was centered again. Because some flow was diverted to move the 3PH cylinder.
In any case, if the 3PH control valve was not centered and introducing a restriction, what would the effect be? Increased hydraulic pressure and heat generated across the restriction for a given engine RPM, until the RPM was increased to the point where the PRV tripped?
Regardless of the direction of flow, if there were a restriction in the hydraulic loop, if the OP has good hearing, might he run the engine up to full RPM and listen to see if the PRV trips? Should not trip if conditions are normal, right?
But in the end, perhaps none of this matters other than for intellectual curiosity. But if the PRV trips with nothing being activated this would likely point towards something other than a linkage issue being the root problem.
Seems like either the linkage is jammed somehow or the shaft the linkage tries to rotate is frozen, for reasons to be determined...