Major Nelson points out in his blog this Wired interview with J Allard.

The article clearly places Bungie's Halo and Halo 2 in the proper pivotal place in the Xbox's history:

Then came Halo 2. Microsoft gave its in-house studio, Bungie, a Hollywood-size production and marketing budget for the sequel. Elaborate teasers ran alongside movie trailers, and Bungie's preview at E3 in May 2004 wowed members of the game industry press. Last November, fans lined up around the block to pay their $50. When the stampede was over, the title had posted a bigger opening day - $125 million - than Hollywood's biggest-ever three-day opening weekend ($114 million for Spider-Man). As it turned out, Halo 2 lived up to its hype. Perhaps most important, it took full advantage of the Xbox's online capabilities, allowing players to trash-talk long distance in real time. In its third year, the Xbox division turned its first profit during what Ballmer refers to it as "the Halo 2 quarter."

However, aside from two oblique references without any specific information, there's nothing about a possible Halo 3.