Hello:
You need to use the Battle.Net App… Call of Duty icon. See my post for more details:
https://us.forums.blizzard.com/en/blizzard/t/not-able-to-call-of-duty-modern-warfare-3/48837/5
There was a News blog last September on Activision’s decision to put all their new games under “Call of Duty”; you can read it here:
Best of luck in your games !