What video card do you have? I assume it's a Radeon since you posted in the ATI sub-forum.
If so, then one of your monitors must be connected using the DisplayPort connection. If your card does not have a DisplayPort then it does not support Eyefinity.
Based on what you described, you do not have a card that supports Eyefinity since you have a VGA port. All Radeon HD cards that I've seen which supports Eyefinity does not have a VGA port.
You either need to replace your current card with a Radeon HD card which specifically states it supports Eyefinity. Or you need to install another video card to connect to that 3rd monitor.