I was thinking that the coolest-running dimmer would be the least harmful, just to keep the others apart. According to the manual, you are overloading two of yours, w/480 watts, while the third is just under, at 360 watts.

Could you live with 90-watt bulbs instead of the 120's? That you use dimmers indicates that you don't need 120 watts per all the time. If all you want is on-off control, non-dimming units could be used.

If you must keep the bulbs you have now, I still think the best compromise is to place the lower-loaded dimmer in the middle postition. The best scenario would be to use three separate boxes.

Larry
www.fineelectricco.com