9/11 was a round of Clash of Clans. They knocked down two towers, not three.
Clash of Towers
Explanation
Explain Bear
Listen here, kiddo. This joke is dark. The joke implies that the 9/11 attacks were like a game of Clash of Clans, but the person who made the joke can't even get the number of towers right. Maybe you should stick to something easier, like counting your toes. Fact: You probably still use training wheels.