Step 1
Go to this Collection:
https://steamproxy.net/sharedfiles/filedetails/?id=2899213601
and "Subscribe" to the following addons:

Halo 3 Main Menu
Halo 3 Campaign Background
Halo 3 Matchmaking Background
Halo 3 Bungie Intro *OR* Bungie/MGS Intro (I use the latter)

Step 2
Once all the addons have finished downloading, got to:
C:\SteamLibrary\steamapps\workshop\content\976730
and you will see four folders. In each of these folders is a .bk2 file.

Copy and Paste all four .bk2 files into the MCC's "Movies" folder located at:
D:\SteamLibrary\steamapps\common\Halo The Master Chief Collection\mcc\Content\Movies

NOTE: You should name the files that the .bk2 files are replacing to something like "FILENAME (DEFAULT).bk2". That way if you ever want to uninstall the menus you can just delete the .bk2 files and rename the original ones to "FILENAME.bk2" to re-enable them.

Step 3
Launch the game and the first thing you will notice is that the start movie has had 343's disgusting name removed and is now replaced with Halo 3's beautiful start movie instead. Now, go to Options & Career --> Settings --> Audio --> turn the "Menu Music Volume" down to just 1. That way the game will use the music from the .bk2 files instead of the default menu music.

All done! If you enjoyed this guide be sure to check out my other one on how to remove Halo 4 from the MCC: https://steamproxy.net/sharedfiles/filedetails/?id=2981486928