When did you first play in Forge mode in a Halo game?

I remember playing Halo 3 for the first time when the game came out in late 2007 and seeing this new "Forge" mode. If I remember correctly, it was the first game mode I played of Halo 3 as I played local multiplayer of Halo 2 before the sequel hit. I quickly discovered it's a mode where you can shift objects around and modify some aspects of a map. Later, in 2008, I got online internet and got the free Heroic map pack with the Foundry map and that might have been the start of me taking in a lot of these creation modes in video games in general. I watched YouTube videos to learn about Forge mode things such as interlocking and geomerging. There's no way I would have ended up the same if Halo 3 didn't have a Forge mode.