(c) Any stoplamp or supplemental stoplamp required or permitted by
Section 24603 may be equipped so as to flash not more than four
times within the first four seconds after actuation by application of
the brakes.
Not sure about this Secton 24603, but there are circumstances where it is legal. I've seen them in Colorado, 3rd brake light will flash like 4 times within a few seconds of applying the brake then go solid.