First person allows deeper insight into the protagonist's character. It allows the reader to identify more fully with the protagonist and to share her world quite intimately. So it suits a story focused on one character's personal journey. However, first person shuts out insights into other characters.

Third person allows a deeper exploration of the relationships between characters. We can see their misunderstandings and hear what they think about each other. We can create a more complex structure with various story threads running parallel.

My advice to aspiring writers of fantasy trilogies or series is that each book needs two main plots. There's the 'big story', the over-arching grand plot of the entire series, and there is the complete-in-itself, one-book plot.

As a novelist, I'm endlessly fascinated by human behavior and interactions.