As someone that has been rear-ended more time than he cares to count, I think a 3rd brake light is a good idea. Of course, I also think there is no hope for someone that hits something as big as a house sitting in front of them, but that's another story.
I was looking through the products on the website that Devoman provided the link to, and found this, I think it would be an excellent addition to the 3rd brake light:
Brake Light Pulsator
Get their attention fast when your brake lights pulsate at 300 pulses per minute (5 per second!). Simple two wire connection - cut the feed wire to the light(s) and install the STOP-ALERTTM with the terminals provided. Works for third brake lights or main brake lights or both. Solid state, uses no additional power. Rated at 60 watts continuous (5 amps incandescent), great for two #1157 bulbs or Ultra Brights plus a 3rd brake light (for Halogen taillights, one per light is needed).
Only 1-3/4" x 7/8" x 1-1/8.