This one is fairly simple. You can play against the same person for the challenge; it doesn't have to be three different people (in case that wasn't clear). Go to multiplayer, set up a two player private match (you have to press
from the multiplayer window to toggle a private match to on) and invite a friend to play. You can then beat up on eachother until you both have three wins. Feel free to boost it. You can even build a red deck with a lot of direct damage spells, turn off simplified targeting from the advanced options menu, and the person who will lose the match can hit themselves with damaging spells to make the games go quicker.
Please also note, when I attempted this challenge my friend actually had to win 4 matches to get the challenge. The situation almost makes me think you have to win three in a row to get the challenge done, but I'm not sure. Someone please confirm this if you can. My friend won the first match. I won the next three and got the challenge. And then my friend actually had to win three more to get the challenge as well. So, I'm not sure if losing a match will reset your progress.
Anyways, as a bonus you also get a booster pack for every win in multiplayer, so it is a nice way to get a head-start on deck-building if you haven't finished the campaign yet or unlocked all the cards. The boosters may not appear automatically after you finish your matches, so press the
button to check your notifications. You can open them up from there. Let me know down in the comments if you have any other questions.