So, I figured out how to complete this challenge after long pondering the wording on it, here's what you have to do (easiest on Hard difficulty as 3* artifacts are fairly easy to get):

1)  Work through Kaerok Castle to get a full 3* Heal set, equip all items to first Champion and then upgrade all artifacts to lvl 8.

2)  Work through Sewers of Arnoc to get a full 3* Offense set, equip all items to second Champion and then upgrade all artifacts to lvl 8.

3)  Work through Catacombs of Narbuk to get a full 3* Defense set, equip all items to third Champion and then upgrade all artifacts to lvl 8.

4)  Go back through the 3 campaign areas above and get 2 artifacts from each (obviously all 3*).  Heal: Weapon and Helmet, Offense: Shield and Gauntlets, Defense: Chestplate and Boots, equip all items to the fourth Champion and then upgrade all artifacts to lvl 8.

Once you do this you have equipped 4 champions with three 3* sets with level 8.  Not sure if you absolutely have to equip the artifacts first and THEN upgrade or if you can upgrade first but I did it this way and it worked.

Happy raiding.

Still couldn’t get this challenge through.... wasted billions of silver already....

All I have left is 6.2 & 6.3. I did 30 attempts this morning bringing my total number of attempts for the breastplate to 130 and I have had FIVE 3 star BPs drop! That's a 3.8 percent drop rate. This means that I have a better chance at getting a legendary from a sacred shard.

I hate to spend all

I have been able to figure this out and it is not too difficult.  Below is what is needed.

1) 4 separate champs must have (3) 2 piece sets equipped of 3 star quality or better.  (In the Champions tab you should have 3 circles under "Set info.")

2) Every single piece under those champs must be upgraded to a 8+ ratings.

Things I missed in the process to make this more confusing.  Literally one of my champs had a 2 star boot on.  Several of my champs had a 4 piece set and a 2 piece set which does not work and only puts 2 circles under "Set info."

Same thing here, i have 5 champs all with 3 or more stars artifacts and all artifact is lvl 8 or more.

Hi men. This is not an error or a trick. I know how to do it right. Here is how to do it. You equip 2 more heroes with 2 sets of offense. Of course this 2 sets re 3 stars and lever 8. You don’t need to find more because y already have 1 set from the beginning of the game,so you equip this purple offense set to the third hero,it work if you don’t equip any items of this set to another hero. If you already equip and remove it the resurt will still 2/4. The easiest way is find 2 sets of offense (3 stars),upgrade to lever 8 all and equip to 2 heroes more. So you finish this challenger easy. I don’t know how to post picture here,maybe you can understand easier.

The piece of information that is not specifically mentioned is that your 4 Campions have to be 1 from each Affinity. 1 from Red (Force) - 1 from Blue (Magic) - 1 from Green (Spirit) and 1 from Purple (Void). I was able to do this both with artifacts that were 3* and leveled up to level 8 both on the champion or leveled up and then equipped on the champion.   Good luck and have fun, remember this is supposed to be fun!! :-)

SOLVED!!!

one champion with 6 ATT manufacts, 3 stars and level 8

one champion with 6 HP manufacts, 3 stars and level 8

one champion with 6 VEL manufacts, 3 stars and level 8

one champion with 2 ATT manufacts, 3 stars and level 8, 2 HP manufacts, 3 stars and level 8, 2 VEL manufacts, 3 stars and level 8

This may help. As I was making sure the equipped artifacts were of the same set, at least 3*, and level 8, I noticed that Shield and Lifesteal require you to have 4 to make one set. Therefore, if you are using Lifesteal or Shield (or other 4-artifact sets), that Champion will not count toward your goal. Make sure you are only using artifacts that require 2 pieces in a set. Even if you have a champion equipped with 6 Lifesteal artifacts, that champion is only sporting ONE complete set. 

-The affinity of champion does not matter

-The sets dont have to be of same type

-The equipment can be upgraded before or after equipping it

The only thing you have to make sure is that you do NOT equip somthing requiring a 4 pice set.....

If you have 3 icons under the (i) Set info, you have the right pices. Then all equipment must be 3 stars.*** not 2 star is allowed **. Then you upgrade it to level 8...