There may be a loose (or intermittent) connection somewhere in the third light, or where it's connected to the track, or on that part of the track, etc...
I suppose it's also possible you got a batch of bad bulbs. You could try moving one of the longer-lasting bulbs from the other fixtures and see if it goes out right away.
Otherwise I would try reconnecting it to the track, moving it in the track, moving it to the other end of the track or another track, replacing the piece that connects it to the track, or buying a new one, depending on your time/budget trade-offs.
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