You need to complete the main campaign levels on veteran difficulty. Walkthrough of the main campaign levels on veteran difficulty with 3 stars:
Gray Ravens:The High Cross:Waterfalls Trail:Redwood Stand:Royal Gardens:Gryphon Point:Rockhenge:Grimmsburg:The Crystal Lake:Neverwonder:The Unseelie Court:The Ascent:Arcane Quarters:Mactans' Retreat:Shrine of Elynie: