Lock out valve is a good place to start. But other than maybe a cut o ring or really bad valve or screwed up adjustment mechanism I can't think of anything else with the 3pt?
Have you changel your trans filter and or cleaned the screen if that's all it has. I would start there and change the fluid if you haven't already. Fluid is like $30 for a 5 gallon bucket so there is no reason not to change it and the filter if it's a new to you tractor to start fresh with fluids.
I picked up some Traveller Premium Universal Trans/Hydraulic Fluid from Tractor Supply today- 5 gallons. Says for J20C, J20A, J14B and Yanmar on the back. I'll do that and the filter this evening and provide an update. Thanks for all the feedback folks!