After much discussion on the forum about cultures and races in Halo, Bungie's Nathan Bitner has stepped in to clear up a bit of confusion:

We never have officially stated (unless possibly misquoted in a magazine or elsewhere) that the Covenant consist of three distinct races. Someone had it nearly right, but it has grown to be quite misinterpreted.

Hmm... the rest of the post sets the discussion back on track with a few hints.

In related news, I'm increasing the forum's message limit to handle days like this. And blazing electric death upon on all you nasty forum posters for giving me so much to catch up on every time I get back from work! You know who you are...