Maybe Im grasping at straws here but, I have noticed a few things that may point to these three being the main characters in the story.
1. On the main poster(which is also the same as the Trans Mask of Fire box) you see these three.
Not that it really means much, but it was just something I observed. I dont know if this means that Gali, Pohatu and Onua are going to take a a big back seat much like Nuju, Onewa and Whenua in the LoMN Movie, but we will see.