The director and star of Creed III, Michael B. Jordan, is more than willing to explain why Rocky doesn't appear in the Rocky spin-off. After reconnecting with his childhood friend Dame, Donnie (Michael B. Jordan) is forced to reckon with the reality of his childhood and the worrisome influence that Dame had on him. As it always does in Rocky and Creed movies, it eventually leads to a boxing match.
https://screenrant.com/creed-3-rocky-absence-explained-michael-b-jordan/