You nominate them on the main page -- either submit them all three at once, or nominate each one separately (note: they may not be chosen in the same time period). I've thought about submitting for years (before there was a Pet Talk, in fact) but haven't done anything about it.

[There may even be a link to the nomination page within Pet Talk. I'll look around and let you know.]

Popped out to peek. Not that I could see, but here is the link for the 'x of the Day' nomination page Good luck.