My 3725 has three positions on the PTO switch-off, link and on. The link position is linked to a position sensor on the 3pt. When in that position the PTO comes on when the 3pt is lowered and shuts off when it's raised. If yours has that and that's the position you are using, check the position sensor and it's wiring. The sensor on mine is on the left side 3pt raising arm where it comes out of the transmission housing.