If it is popping the reset button..more than likely a bad solenoid on zone 3.....or direct short.
Locate zone 3....remove wires from solenoid....try again with timer. With the solenoid essentially now removed from the system....it shouldn't pop the reset button. If it does, it isn't the solenoid, if it doesn't...replace the solenoid, should work.
Good luck,
Sprinkler Solutions, Inc.
Arizona and Colorado
www.sprinklersolutions.net