Halo MCC content not installed when it is

I'm Samy an independent advisor. Welcome to the Xbox community. How are you?

These steps can help you, give it a try.
Check for updates on Windows Update, update everything including optional/additional updates.
1. Logout from Xbox app and Microsoft Store app
2. Look for the Xbox app and Microsoft Store app in the Windows menu, right-click, click app settings and then click terminate, restore and reset. Do this with the game too.
3. Clear the store cache type ''wsreset'' in the Windows bar, run as admin, and wait for the screen to close and open the store automatically.
4. Open the Microsoft Store app, click on the 3 dots in the upper right corner>downloads updates>get updates and update everything.
5. Restart your PC, log in to both apps and see if that solves the problem.

To those having install issues, do as other have said, make sure to update halo mcc and make sure you have the disc in when installing, the campaign resides under the halo 3 campaign in halo mcc.

Thanks for contacting us my name is Samy, I am advisor independent and I will be helping you today.

How are you, mate?

These 2 games are DLC's for Halo MCC, you have to purchase and install Halo MCC to be able to access this content in-game.

Halo 3 ODST is an add-on for the MCC so it needs the MCC installed as well. Have just checked the Store and ODST can't be bought separately so it must be the MCC add-on:

https://www.xbox.com/en-GB/games/store/halo-3-odst/9N007VX9RGXZ/0010

As you will see on that page:

"This content requires a game (sold separately)."

Game meaning MCC

I've had something similar happen myself and was going to suggest the same thing which it seems fixed your issue. At least now you know for future issues in case it happens again. Enjoy the game and happy gaming!