The drain plug is at the bottom of the housing on the right side. It is near where the right side draft arm for the three point is attached to the transmission housing. You will need a 1 1/8" wrench. The filters and oil is definitely where you want to start. It can absoultely be your problem, but there are other things that can be wrong also. Since your three point leaks down(how quickly?), it is likely it needs new orings and seals installed in the valve and cylinder. The steering valve could also have blown orings that leak internally and rob the system of oil flow. Also transmission hi-lo(rabbit and turtle lever on left side of steering wheel) or pto clutch can have internal leaks, hydraulic filter relief valve can be bad, transmission pump or main hydraulic pump may be worn. Try starting the tractor and leave it sit for maybe 15 minutes. Then try feeling all the hydraulic components to see if anything is hot. Heat build up indicates an internal leak.