stands for the year 3000. a jonas brothers song appearing on their first and second albums (its about time and jonas brothers)
this was first introduced by band member, nick jonas, when writing in order what songs were going to be played for a venue. ironically enough, nick doesn't like it when people abreviate the titles of songs. but, apparently, he likes y3k enough to call it such.
"dude! the world won't end in 2012! the jonas brothers have been to the y3k!"