1999 Jeep Cherokee brake lights

Electrical problem
1999 Jeep Cherokee V6 Four Wheel Drive Automatic 106777 miles

My brake lights have not been working properly for the last few weeks. The lights work fine when my headlights are turned off but as soon as I turn them on only my third brake light is functional. I have checked the fuses and they look fine so I am wondering what else it could be. I don't know much about cars so I don't know if this is something I could fix myself. I'm trying to figure out what the problem could possibly be and a ballpark of how much something like this might cost to repair.