A few characters have already been confirmed to return in Frozen 3, but there are others from the first two movies that should make an appearance as well. Of course, the third installment in the Disney franchise would be impossible without the likes of Elsa, Anna, Kristoff, and Olaf, but they've met others throughout their adventures that are nearly as important. Additionally, characters from the past, such as Arendelle's royal family, have been pivotal to the plot of Frozen and Frozen 2 through various flashbacks and should therefore continue to be a part of Anna and Elsa's story in the future.

https://screenrant.com/frozen-3-characters-need-to-return/