No, the device measures the total wattage that the PC is drawing. You could assume that all or most of the wattage is drawn by the 12v rails. Subtract a bit for psu inefficiency, and you could get some idea of the amps used.
I do not think tri GTX480 will be very useful, possibly so only if you are gaming with a 2560 x 1600 monitor.
If you are willing to drop that much cash on your graphics cards, a top psu will be a drop in the bucket more.