Yes, Call of Duty: Modern Warfare 3 has full cross-platform support. This means that you can squad up with players on any console PlayStation, Xbox, or PC and enter the battlefield together. This is true for any online mode in the game.
The website crash may occur because of a caching issue. In this case, users can access the site, but it fails to load on your end. To fix it, clear the browser cache and refresh the page. Alternatively, change to a different network.
There are several reasons a website can crash, including broken code, malicious attacks and hosting errors. Another big reason websites can go down is massive traffic. Every website can crash, but knowing what caused the crash can help you fix it quickly.