In Gears of War 3 for the Xbox 360, you get MVP ribbons by completing a versus match with the most points.
If you’re playing online, it’s obviously going to be much harder to get MVP ribbons, unless you’re really good and dominate each match your playing.
Luckily, you can get MVP ribbons during Private matches as well against AI bots. If you want a lot of MVP ribbons, do a Team Deathmatch game with low respawn rate against casual bots and single round matches. You’ll be getting tons of MVP ribbons before you know it.