Check each bulb. Yes. Check each bulb. Flick each one with your finger and watch the filament. The one that fails the flick test is the one that is about to go. Learned this from one of the many brilliant diy'ers on this forum.

Could also be the third tail light is loose.

Also. Buy only the oem bulbs. I forget the brand at the moment (Sat. night at 8:00 p.m. hic.)