Have you tried to go into the server browser and selected a map from the Aftermath DLC expansion like Epicenter, Markaz Monolith, Azadi Palace, or Talah Market?
Please also make sure that you have Multiplayer Update #6 and the Aftermath DLC downloaded in your Xbox 360's storage device before attempting the above. You can check by going to Settings > System > Storage > Hard Drive > Games & Apps > Battlefield 3.
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