A number added to the end of a username or gamertag, usually (if not always) by Japanese gamers (or non-Japanese Otaku) to represent the respectful honorific "-San" which sounds the same as the number three in Japanese.
Usually used by Girls, Catfish, Weird people. Mainly used by weird people and Catfish, you can tell its a catfish when they do the :3 in every sentence.
Girl on Whisper: Hoi!!!!!!! :3 im 16 :3
Guy: Cool
Girl on Whipser: Wyd :3 ????
Guy: nm
Girl on Whisper: Okay :3 :3 :3
Guy: Alright you can stop catfish... you're probably like 34
"Girl" on Whisper: Uhhh :3 ... Yeah you got me, and no i'm 56.