Hi,
Modern Warfare 3 is included as an option in “Call of Duty”… the 1st screen shows the installation folder and game language and some other options. The 2nd screen shows what will be installed… except for the Base Game, I think everything else is optional. For example, if you don’t need Warzone, I think you can deselect it.
Click on the arrow to the left of MW 3 and ensure “Campaign” is selected… then click on “Start Install”
Good luck in your games !