As Slash kinda hinted, chainline might be part of the problem. Mountain triple cranksets tend to sit a little further out from the bike's centerline, so a road derailleur might have difficulty reaching the outer ring. According to Sheldon Brown, typical chainline for mountain bike triples is 47.5 to 50 mm. Shimano spec for road triples is 45 mm. (Chainline is measured from center of seat tube to the middle ring of a triple crankset.)

Do you have enough clearance between your inner chainring and chain stay to move the chainring in a few millimeters? If so, that may be the easiest fix, depending on what kind of crankset you're running.