Bicycle Mechanics - adding a front derailler to a folding bike

I am sure i posted this question in this forum before but I can't find it now :(

I want to put a hub gear on the rear wheel of my 20" folding bike and would like to add 3 gears up front to give me a better spread. I am hoping that by carefully choosing the cog in the back and matching it with 3 gears up front I can keep the chain up high off the ground. I have looked at some 20" bikes with rear deraillers and some of the chains look like they are only 2" from the ground - too low.

Is it possible to do this? I want three gears up front.

It is possible as long as the derailleur doesn't interfere with your folding. Bike Friday makes bikes with front derailleurs. Working out the cable path is necessary, and the derailleur will have to be able to travel as a unit with your rear triangle and drivetrain when you fold the bike; that is possible if the seattube and the rear triangle don't fold relative to one another (e.g., as in a Swift or Bike Friday) or be located so that the chain path does not need to leave the derailleur cage when folding (e.g, with a Brompton.)

Thanks! Are there "clamp on" front deraillers I can buy? I don't want to drill or weld...I have narrowed the choices to a few bikes and most are aluminum - one is Chromoly. I want to leave the frame alone.

Yea... many if not most front derailleurs are clamp on.

I want to put a hub gear on the rear wheel of my 20" folding bike and would like to add 3 gears up front to give me a better spread.

Mixing front triples and hub gears aren't as straightforward as one might believe, and can easily leave you with repeated ratios. Do some tinkering with Sheldon Brown's gear calculator to determine the consequences first.