Are you running the 3 monitors as an Eyefinity group or independently?
Eyefinity SLS (single large surface) means taking x number of screens and treating them as one, giving each an equal portion of the resolution and making a continuous desktop. So each of the monitors would most likely be operating at 1680x1050.
However if you never created a display group with 1 large resolution, then each of your monitors would be acting independently as extended desktops with their own resolution. At that point you should be able to change the resolution of any given monitor via CCC. Keep looking.