I realize that the diodes I suggested won't work either, since obviously a single turn-signal will then cause the 3rd light to blink.
You could use a flip-flop circuit that would operate the 3rd light when both stop lights are on, and then doesn't turn it off until both lights are off. For example, an AND gate to SET the flip-flop and a NOR gate to RESET the flip-flop. (Or since some FF SET and RESET are active low, use a NAND gate for the SET, and an OR gate for the RESET). That way the 3rd light will still stay steady on when the brake light is maintaining one light on and the other light blinking, but stays off when the brakes are off and either light is blinking.