https://wccftech.com/halo-3-odst-pc-available-firefight-xbox-update-notes/

Halo 3 ODST PC is now available for download through a new Halo MCC update that also includes ODST: Firefight on Xbox One.

After months of waiting, 343 Industries has now launched Halo 3: ODST on PC as part of Halo: The Master Chief Collection. Arriving alongside the stand-alone Halo 3 expansion is the wave-based survival mode ‘Firefight’, which has been added as part of the ODST DLC package to Xbox, Steam, and the Windows Store.