to 3* a campaign stage you have to:

Use no more than 2 champs

They must both be alive when you beat it

Clear it in under 10 minutes

I ran a single Champion Level 40 and completed the first three locations in under 15 seconds for each stage, including stage 7, without the champion dying. I do not understand the Yellow stars vs Blue stars.

Yellow star: completed goal,

blue star: Not completed yet

You want all stars to be yellow

You get 1 yellow star for beating the satge.

You get the 2nd yellow star if you beat it with everyone still alive at the end

You get the third star if you beat it with only 2 champs (or solo), everyone alive at the end, AND did it in under 10 minutes.