According to Shimano the Shadow is compatible with 11-32 or 11-34 only.

With a fixed upper knuckle the angle is set for a specific derailleur path and will give less than optimal performance with anything other ratios. The only Shadow derailleur that is adjustable from road to mountain cassettes is the Saint, but it won't handle a triple. I'd go for the Ultegra, or if you can find one, a pre-Shadow XT.