This is another way to say the gang called the Crips. It represents that by the 3 meaning C, the third letter in the alphabet. The rest is pretty simple to figure out at that point.
Guy 1: "Yo, do you wanna join the 3 18 9 16 19?"
Guy 2: "What is that?"
Guy 1: "You have to join it then I'll tell you"
Guy 2: "Fine, sure I'll join"
Guy 1: "Why would you want to be apart of the crips that's so dumb"
Guy 2: "I hate you"
when you're holding up your phone and making faces at it, as though you are taking a selfie, but you're really taking a picture of the person across from you or the wall or anything else that seems interesting but you don't want to be caught dead taking a picture of.
This action is often made more convincing by wiggling the eyebrows or opening the mouth, to pretend you're trying to get a Snapchat filter to work.