It could be the seals as Rich mentiones, and that is usually the case, but there are two other possibilities. One is the control valve spool leaking. The other is the safety relief valve on the control valve leaking. The good news is that there is a very easy way to check if it is in the valve/safety, or in the cylinder seal.
Lift the 3PH up to a position where you have observed it leaking down from before. Once there, reach down in front of the seat and screw that 3PH speed regulating valve handle that 20/20 mentioned fully clockwise/closed. When fully closed, this valve prevents any fluid from returning from the 3PH lift cylinder to the valve. If the 3PH dosn't leak down with this valve closed, the leak is in the valve or safety is stuck open. If it does leak down, the problem is with the seals. If you have any weight on the 3PH, you may have to lift the weight off the hitch(lift impliment with a jack, or back the impliment onto a rise/bank to lift it) in order to unscrew that speed reg valve, as weight on the hitch can hydraulically lock it closed.
You should be able to withdraw the piston thru the rear of the 3PH lift box by removing the rear plate/topliink bracket. You will of course have to drain all the hydraulic fluid, and as mentioned this is an excellent time to clean out the hydraulic system. Don't be surprised if you find the bottom of the resovoir covered with metal shavings I did on my brand new machine... On a scale of 1-10, this repair is probably a 2-3, but I have been doing this for a while. No real special tools needed, but a set of drive up ranps to put the front wheels up onto will make draining the resovoir easy. The resovoir drains from the rear, from the lower left hand drainplug(looking from the rear). It dosn't jump right out at you as it is in line with all the other mounting bolts...
Besides dirt, another issue that might kill the piston seals is a misadjusted linkage. The linkage that connects back from the lift arms to the 3PH control valve is supposed to stop the 3PH lift cylinder at the top. If this got misadjusted, the piston may travel too far inside the lift and rip/tear the seals on the edge of the cylinder. There is a little raised piece on the inside of the rear cover that is supposed to stop the piston travel at it's extreme, but this may not be configured correctly. I seem to recall a discussion here on the lift control linkage and someone suggesting finding the highest safe spot that the 3PH can achieve without damaging the seals, then setting the stop on the linkage and then cutting off the excess threaded rod to prevent the thing from being misadjusted... This would be easy to determine with the lift rear plate removed while you are fixing the seals.
