Get a hacksaw with a 32tpi or finer blade. As you're planning to cut off three inches, practice by cutting off one inch first. After that you'll have enough experience to cut it at the right location. As others have said, I usually tape the area to prevent accidentally sawing the steerer surface. When I begin cutting, I use light, small strokes until the blade goes into the carbon a bit. I repeat around the whole tube before I start to actually cut. Most important thing is not to force the blade if it gets "stuck" otherwise you risk delamination, cracking, etc.