Where it comes from
In Ep. 84 they say 'we're three alvins' outright.
How the hosts use it
Used as a self-nickname for the trio, usually in a callback after a listener comment or a bit of role-sorting. They argue over who is which chipmunk (Simon, Theodore, Alvin). Ep. 95 adds the related 'seth's away so the alvins can play' and 'three Daves' comments, so the fans' own role-casting is part of it. Seth is the Dave, the human who yells at them.
Written by the AI from the transcripts. Every sentence was checked against them, one by one (the full episode transcript wherever a sentence names one); 6 sentences that failed the check are left out. How the check works.