I think you mean brake lights. Two things to check (or have checked) is the brake light switch and the sig. light switch.
Either one may be defective. The brake lights work through the sig. light sw. If 4 way flashers work then wiring and bulbs should be ok.
The third brake light can still work when others dont. As i said if the flashers work no need to check wiring sockets or bulbs they are ok!
Just be sure you have checked all fuses.