Without listing your current group attempting to 3 star this mission its going to be extremely difficult for us to help. 

Levels, abilities like do you have a reviver, crowd control and all that. 

Post your team and wait for a better response as we've all been there.

yes give us the exact mission ur doing please and we will let you know if ur unlucky or doing it wrong.

just luck, i'm afraid. the drop rate for 3* artifacts is pretty horrendous on normal (i'm assuming you're on one of the challenges that requires normal difficulty); just under 7% chance. there's not really any way to get through it other than grinding.

i can tell you that the number of stars you get has no impact on your reward, so there's no reason to 3* it everytime. you're better off filling your extra slots with champions that need levelling.

This unfortunately.  Drop rates for 3* artifacts from Normal campaign are pretty low and it's pure RNG.  As mentions definitely get some food champs in there and at least get some additional leveling done at the same time. 

got it eventually, thanks guys - got a good few commons upgraded in the process - also spent the effort catching my progress missions up to where I am,so when i have to grind stuff for the missions it's roughly at my current level, rewards per turn feel much better that way....