How to setup triple front derailleur with a double crankset
As with many things bike related, it might not meet the specs, but that doesn't mean that it won't work.  

Some questions that will help answer the question:


Are you using friction or index shifting for the front?  If friction (such as bar-end shifters) you should be fine.  If indexed, you might have incompatibilities with cable pull, especially if you're trying to use the same shifters or switching between a road vs. MTB derailleur.
Do the limit screws allow adjustment to the range of motion that you require?  If so, you might be OK, but it really depends on the number of teeth on your derailleur.


If you happen to already have the parts and have worked on bikes a bit in the past "just try it" usually works for me.  You might be pleasantly surprised.  If you have to buy the parts or pay for labor it might not be worth the trouble.

Far more information on the topic can be found here:

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