Each Challenge missions yields up to 3 stars.

If you don’t earn all 3 stars on a mission, you can replay it with no cooldown as many times as you want to get the missing stars. However, you can’t go back to the previous round once you advance to the next one.

You also earn additional stars for completing full sets of 6 missions on a given difficulty.

You can see the number of stars you’ve collected in the lower left of the Challenge screen.