Join Date: Oct 2009
Posts: 4,279
Make sure your card supports Eyefinity (required for 3+ monitors @ max res afaik). If it doesn't, you're probably out of luck.
If it does, and since I couldn't find a 360 with native VGA, my best guess is you're using an Eyefinity incompatible VGA adapter, ie. a non-active one.
I never bothered to figure out what "active" means in this context - all I know is an active DP-DVI adapter (about 3x as expensive as a standard one) was required to enable triple monitor support with my HD7850. I might be wrong since this is VGA we're talking about, and your card is newer, so best take it as a clue for further research.
HTH, GL