On my experienced the best method to get 3 star on campaign is using characters with AoE skill attack type and life steal equipment set. I use 3 star champion Elhain and it is very to beat campaign. After my Elhain reach level 60 i'm even can beat campaign deadlands brutal difficulty with solo play. This helps me a lot to farming exp and complete the champion training event. 

I added a champion of the opposing affinity to the dragon's.  I also upgraded all my gear.

Good luck!