Hello Dana,
Although I am by far no expert here...
it makes me think that for this type of side markers/reflectors without illumination,
only two holes (as on your passenger side) should be sufficient for the two scews necessary.
The third hole does not make sense to me. Maybe that one was accidentally drilled 15 years ago after respraying of your car?
But that one is covered by the side marker reflector anyway, isn't it?
At least that's what your first pic (from top) suggests...
IMHO...
Achim