In Halo 3, the Forerunner installation where the final battle against the Flood takes place is the Ark.
The Ark is a massive Forerunner facility located outside the Milky Way galaxy. It served as the construction and control site for the Halo Array, allowing the Forerunners to activate the rings remotely and preserve life beyond their firing range.
The Covenant reaches the Ark through a portal near Earth. Master Chief, the Arbiter and their allies fight the Covenant and the Flood there, while Truth attempts to activate the entire Halo network. The Flood also arrives after the Flood-controlled High Charity crashes onto the installation.
The Ark creates a replacement Installation 04 after the original ring is destroyed. Chief activates the incomplete replacement to stop the Flood, but the resulting damage sends him and the Arbiter into a dangerous escape.