http://sheldonbrown.com/front-derailers.html

i think your 48-40-32 would work very well with a double.

you encounter the biggest problems when you try using a triple FD when your chainrings are close in size...like your current 8 teeth jumps. the bottom of the inside of the cage is going to rub on the 40 when you are in the 48. you can raise the FD so it doesn't touch, but of course you know the implications of doing that when shifting to the 48.

triple FD are great when you have huge jumps like on MTBs or a tourer.