Can Bard's Q stun more than 2 enemy champions with a single cast?
Solution 1:
Upon looking at the League of Legends Wiki (mainly to see if his passive changed), there is not a way for him to stun more than 2 enemies with a single Q. You should be able to progress by stunning 2 champions.
This is just conjecture but maybe in a future patch, collecting chimes gives additional effects to his spells. I am only ruling this a possibility since he has fallen out of favor recently and Riot wants to give him more scaling without effecting his early game. Again though, that is just a guess based on the wording.