In 23rd episode of Kanon 2006 anime, Yuuichi goes to School Ayu visited and met her there. Yuuichi offered to grant her third wish.
She wished for Yuuichi to forget her, but he declined to grant that wish because he is not able to.
Ayu then realizes what the third wish is, but we only see her mouth moving.
What did she wish for?
The simple answer is that we don't know.
However, in Episode 24, Shiori makes a guess (based on a feeling) of it being, "I want my dearest person to always be smiling."
"For that to happen, everyone around that person has to be having a happy life, too." Which brings her to the conclusion that such a wish triggered all the miracles happening around Yuuichi, such as all the girls getting better, including Ayu herself.
