>>11945837 (OP)
If I remember correctly the E3 demo was almost entirely scripted and made for the show.
Tons of levels were cut. Halo 3 was basically the stuff they wanted to have at the end of 2 but stretched out.
More content in the Quarantine Zone like the "Forerunner Tank" which was a big forerunner factory/war machine if I remember the concept right. They talk a lot about it in the commentary. Great video.
https://youtu.be/TAW2F6oPYMc?si=Bdlgq4WD9w_zwZMj
The sad part is Halo 3 even had to have some big cuts. Some highlights being a forest area on the Ark with unique "Guardian" enemies (this is what the Guardian multiplayer level comes from). "Cortana" was pieced together from what was part of the latter part of Floodgate, inside the crashed ship. What was originally Cortana was more fleshed out and was meant to have setpieces like a battle in the Mausoleum of the Arbiter against infected former Arbiters and a boss fight against Gravemind.