Can someone explain what 3x3 flush exactly mean

Just got my son a 2007 Acura TL Type S AT with 127k. So far he already put 1,200 miles on it since then. I’m gearing up on the next fluid change. Engine oil and transmission once it hits 130k. Thinking of going Amsoil. What’s this 3x3 I’m hearing about? He’s also complaining about his gas mileage 🤷🏽♂️. He’s not getting great gas mileage lol. His dad probably hitting vtec all the time whenever he’s drives it 🤪, ok ok I don’t drive his car much. The car is a little slow considering I’m used to driving V8’s lol. I just want to make sure this car serve him well for a very long time. Any input is highly appreciated!