It is not clear whether Meryl Streep would be able to come back for a third Mamma Mia! Obviously, her character, Donna, is no longer with us, but the franchise has already proven adept at utilizing flashbacks to tell a story and, truthfully, it’s hard to imagine a Mamma Mia! movie without her.
Article source: https://www.cinemablend.com/news/2548598/could-mamma-mia-3-be-on-the-way-heres-what-a-producer-said