Actually, "Triple twenty" means you landed a single dart in the triple section of twenty. Which is the second green circle, in the centre of the 20 area (Not the dartboard).

Oh, just read your question again. It means that your dart landed in the center (small red circle) 3 times in a row.

I don't think "tripe twenty" is a real phrase. I think you mean "triple twenty". It comes from the game called darts ( https://simple.wikipedia.org/wiki/Darts ). In this game, the biggest score you can get in one go is to hit 20 (small red circle in the middle) three times. That is what's meant by "triple twenty". In everyday language, it's used to describe something amazing that someone did. I have never heard people actually use it, but for example, if someone throws something from far away into a garbage bin and doesn't miss, they could yell "triple twenty", I guess...