Samsung Apps is the Samsung App Store that is pre-installed on Galaxy SII. Launch Samsung Apps and find Modern Combat 3.
For Custom ROM users:
http://forum.xda-developers.com/showthread.php?t=1124396 <— Open that URL, and click on "Bloatware Removed" (a link at the top of the post). It has the Samsung Apps apk if you need it. The ROM linked here is Cognition 777 for Galaxy SII.
Little more info on the custom rom situation would be appreciated. Do we need to install all Samsung apps from xda etc