In an interview about Halo 3's upcoming Mythic map pack with IGN, Bungie employees Lars Bakken, Multiplayer Designer, and Brian Jarrard, Community Director, have revealed that they have no plans to put out any more Halo 3 media (expansions or map packs) after Halo 3 ODST and the Mythic map pack.
Jarrard stated, "we are viewing the ODST campaign and the included multiplayer maps as the final exclamation point in our ongoing efforts to extend the Halo 3 experience. We're really grateful to our fans for standing by Halo 3 for so long and continuing to show us such strong support."
The expansion for Halo 3, Halo 3 ODST, will be out later this year and will bring hours of new content to the game, as well as new maps and achievements. It will not be required to own Halo 3 in order to play the expansion.