Earning a three star rank for a dungeon requires a very high score. To do this, build a large combo. When you kill an enemy, a score multiplier will appear at the top. A blue bar will appear below it and begin to drop. Kill another enemy before the blue bar disappears to increase your combo multiplier by one. This will score your next kill with extra points. Continue attacking and when an area has been cleared out, immediately move to the next one.