This is one I have to question," the clutch is stuck". Usually the clutch is engaged all the time when sitting unless you put a block of something to hold the clutch in while it is stored.
They usually will not let you put it in gear when the pedal is depressed, the clutchplate is stuck to the flywheel.
You mentioned that it sat for 3yrs. When you depress the clutch do you feel freeplay? Or does it just drop freely. Did you pullstart the tractor to get it going. To me it sounds like the splines or the center of the clutchplate has been wiped out.
On some tractors there is an inspection plate you can look in. If the tractor was stored with the clutch in, might be that the pressureplate fingers have rusted up some. I can't imagine they would overpower the pressureplate springs though.
Be sure to let us know what you find, this one is a real mystery. One other thing, If it is a twostage, does the pto work? If it does, the tranny input shaft might be sheared.
