SOUNDS LIKE MAYBE SOME ONE HAS REWIRED THIS ONE.
the two outside pins of the switch is for the reverse lights. one wire should be colored black= 12 volts with the key on, the other should be a black wire with a white line(tracer) on it- this will go back to the rear reverse lights.
the centre is the grounding one for the start relay- only in park and neutral will this pin be grounded to the tranny case.
color=brown with a yellew tracer.
in the tranny is a lever that only engauges the two outside pins if it is in reverse. same for the neutral one.
so take the plug off and check the two outside pins with a ohm meter to see if it only engauges in reverse- get some one to help you move the gear level as you check it.
Hope this helps!!!!
JOHN Mac