This Army

Reposting from one of my replies to increase visibility as this might help people get their achievement MUCH quicker :)

Turning in OTHER assault quests while having a champion selected will give credit for that champion. So in order to get credit for 3 champions during a single assault you need to:

1) Pick up "You and What Army" quest and select a champion
2) Complete and turn in any OTHER assault quest
3) Abandon "You and What Army" quest and reacquire it, picking up a different champion
4) Complete and turn in yet another assault quest (other than "You and What Army")
5) Repeat for last champion (you can finish with "You and What Army" this time)

Edit: 11/07/2021

Additional information based on one of the comments below (credit to Duqjin) - for those who want to absolutely maximize their achievement progress on a single assault and don't mind waiting an extra day to complete the event.

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• You can get credit for 4 champions during a single assault by utilizing a total of 4 assault quests and waiting one day for new set of champions (the set of champions changes daily)
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• If you do this, remember to turn in "You and What Army" as the last quest with 4th champion, otherwise you won't be able to switch champions anymore
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Edit: 21/07/2021

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• The quest is available again on European realms (not sure about US) so if you got credit for at least one champion last time then you are able to complete the achievement today by utilizing the above tactic (maximizing with 4 champions if necessary)
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• I completed the achievement today on my character (as I'm sure many people did) so it's definitely possible and I suggest everyone to do the same before they change the way this works (pretty sure the current interaction is unintended)
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Actually buged coz its obviously not a way devs want us to do this.
Achievement description say complete you and what army quest with helper but...
If you pick a quest, chose helper and do another quest from assault and then you will turn in this another quest you get profit for this achievement.
Then you can abandon quest You and What Army and pick another helper, do another assault quest and get profit for another helper.
In that way you can do 3 helpers in one day.

Set of champion changes daily. Today I have 2 new ones available to pick up. So you can get 4 champions per assault in total. (4 quests for each champion as it is described below).