It means the KKK (Ku Klux Klan) which was an American white supremacist hate group. The reason 3 11 stands for 3, 11s with 11 being a K, the eleventh letter of the alphabet.
Guy 1: Yoo how do we talk about the KKK without sounding sus?
Guy 2: How about we come up with some secret code with letters from the alphabet and numbers?
Guy 1: I guess we could do 3 11.
Guy 2: How does that mean KKK?
Guy 1: 3, 11s with 11 being the eleventh letter of the alphabet.
Guy 2: Oh that makes sense. Speaking of which should we join the 3 11.
The best time to sneak into the back of your school to go sledding. However, before sledding, you must perform satanic rituals and sacrificea squirrel. Once that is done you are free to sled if the back of your school has a hill.
Rule: you must meet at exactly 3:11 a.m. near the closest tree you could find
Person 1: Hey, do you want to meet behind the school at 3:11 a.m.?
Person 2: Of course! Don't forget to bring your sled!
Person 3: Let's meet at this exact tree at exactly 3:11 a.m.
Dick. D is the fourth letter in the alphabet, I is the ninth letter in the alphabet, C is the third letter in the alphabet and K is the eleventh letter in the alphabet.