Three years time for testing a turbine overspeed protection is too much. It must be checked atleast twice a year.
While testing, we actually set overspeed trip setting to 104% maximum(actual trip setpoint is 110%) to make sure that turbine is not subjected to unnecessarily high vibration & I think that its enough to determine the integrity of overspeed tripping.
While testing, we take a close look of turbine vibration on all bearings.