To unlock them all, you have to play the mode before it. If you have 3 and the last is blacked out, you will have Rumble and Skirmish is blacked out. Just play one of each mode, then the last one will unlock.
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Haha I meant skirmish.... But thank you
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I had the same problem unlocking it cos I HATE Rumble. So I kept quitting out of it, never unlocking Skirmish.