:3 means that the sender wants to suck the recipient balls deep. It comes from the emoticon combination of :D and c==3 and when combined, :Dc==3 to :(=3 finally having :3
:3 is a representation of a cute/smiling expression and is commonly used to show happiness,/playfulness. Its appearance resembles a sideways cat face.
the "3" being its mouth and the ":" being its eyes
":3" is a commonly used emoticon used by gay femboys. usually to indicate love for someone or just to be quote "silly quirky". this is a very dangerous virus and can cause temporary or even permanent femboy behavior if in contact with said emoticon. if you see this ":3" please stay 8 feet away from the user.