Peter and Susan Pevensie didn't return to Narnia for The Chronicles of Narnia: The Voyage of the Dawn Treader. The third Chronicles of Narnia movie, The Voyage sees siblings Lucy and Edmund Pevensie, along with their cousin Eustace, traveling over the seas of Narnia, and many viewers wonder why Peter and Susan aren't with them. The first two Narnia movies centered on all Pevensie children. Peter and Susan, however, only appeared briefly in The Voyage of the Dawn Treader after they were told by Aslan that they would be unable to return to Narnia after their departure at the end of Prince Caspian.

https://screenrant.com/chronicles-narnia-voyage-dawntreader-peter-susan-no-return-reason/