QAT, QIS, OXO, ZAG, AHI, JOW.
If you don't know these words, you will probably be killed during a tournament Scrabble game.
I tell people that if you learn all the three letter words that are in the official tournament word list (there's only 1015 of them), you will increase your average score by fifty points per game. That's average score.
It took me three months to learn them. But that was the first scrabble studying I did.