How do you get 3 stars in kingdom rush

In each campaign mission you have 20 hearts, you will get three stars even if you finish the mission with 18 hearts. In the heroic and iron challenges you only have 1 heart and if you complete the mission you get 1 star each. Hope this answers your question.(Psst! in the top of the menu screen you have the follow on facebook and twitter, awarding one star each)