On: Airlines are running out of 4-digit flight numbers

[flicken]

The "AAA" is a typo, they probably mean "AA777" for American Airlines 777[1].

A far as why the number 777 is amusing, it's gambling. The combination 777 is a jackpot on a slot machine, Additionally AA777 is a great hand (full house) in poker.

[1] https://www.flightaware.com/live/flight/AAL777