This issue has been brought up countless times and been debated on numerous occasions. The general idea is this: Two clownfish will mature in the tank as the dominant pair. Clownfish are territorial, and will typically hunt down the others and kill them (one, two, three...). Sporadically, they will allow a third fish, or sometimes even another pair to live peacefully. This is not the norm though. Also, keep in mind success is measured in years, not months. Many on here are saying it may be ok, but haven't fully disclosed the number of years they have had this work, or how many might have been killed in the process.
I go by this rule of thumb: If you have to ask, you don't have enough knowledge to make an informed decision.
What will you do when aggression starts? Do you have a Qt setup and cycled you can put it in? What would you think if you come home from work and only have two fish left? You would be knowingly putting a fish in a high risk situation. I personally wouldn't play the game of Russian roulette with my fish, and can't support others doing the same.
Listen to Todd. He has many many years of experience under his belt, and his opinions should hold more weight than most on here (including my own).