My shingles are three years old and I would like to put a metal roofing

Metal Roofing has been sucessfully applied directually over shingles. Problem is uaually the substrate is not as thick as it should be for the fastners to hold. It is recomented that screws to hold metal roofing should be into one inch solid wood. Purlings to fasten the metal can be aplied over the shingles and fastened into the rafters useing long nails or prefered screws. If metal is applied directly over the shingles with out purlins longer screws should be used and fasted through the top of the rib if exposed fastners. If conceiled panels screws should be closer together

Metal roofs do not make condensation. They can exacerbate the issue is slightly if the ventilation is not correct but that should not deter you from running the roof that you want. Make sure the insulation and ventilation are done properly and you can successfully use whatever roof material that you want. If you can post up some pictures of the roof in question, we can give you some more feedback. In many applications, installing the roof over a set of purlins will provide for an over-deck type of venting and will largely negate the potential for any sweating.