I think the best solution (personally) is the hazard flashing during hard braking, which gives drivers a bit more notice that you're stopping pretty rapidly.
The third brake light blinking isn't a good solution. I did this to my wife's Subaru Legacy while we were in Korea because drivers there are as attentive as moldy rocks. After three years, I had to remove it and also replace the LED element because the constant rapid/slow flashing of the brake lights caused the LED's to burn out prematurely. Never doing that again. Instead, the high speed braking which activates hazards is probably a better idea. After all, that's my typical reach (hitting the hazard button) if I have to slam on the brakes in congestion when stopping on the highway from speed.