There has been no error in the series throughout, the author has not tried to add anything extra to the story rather he has continued with the same characters and the mission also remains the same. All the three parts are connected in a compact way and whoever has left any of the previous two might not be able to understand what is going on in the story.
The two characters are gathering an army to reclaim their land and along with it, they are sharing feelings of the kind that were there in Heartless. Different things are happening at the same time in the story which reminds something about Cress to the audience. Narrators Dan Bittner and Rebecca Soler started the series and they are given the honor to end it in the same pitch and style. The two narrators have contributed a lot in this series and their services cannot be sidelined.