I personally love trilogies as long as there is a concluded arc AND a continuing arc in each book. THE HUNGER GAMES is a great example. It was obvious that there were two arcs: Katniss' personal arc in the actual Games and the arc of the imminent rebellion that is out of her control. What was in her control was resolved, while the rebellion happened anyway. That is what I consider to be a satisfying end to a book in a trilogy.
DIVERGENT did not do this for me. Each ending felt discombobulated, and I believe this is because Roth focused so much on Tris' little love affair in the first book and not enough on the macro-level problem that carries the story through more than one book. I feel like each book should have brought conclusion to an inner battle in Tris, and the conspiracy should have bridged to the next book. Instead both arcs were left open and unresolved, which made it an unsatisfying read for me. I think the last book REALLY suffered from Roth's lack of attention to the dystopian conspiracy in the first book. The series felt poorly weighted, like the first book was all smoochy-smoochy love and the last two (especially the third) were playing catch-up, trying to sell us a near-unbelievable premise without having used the first book to slowly build up our ability to suspend disbelief. I mean, seriously, a murder gene? When nothing related to that level of science has been hinted at? People secretly observing a city that is really an experiment? With next to no foreshadowing that the entire world wasn't just like Tris' city? A random ass experiment with the lives of human beings with no setup as to why a government would believably do that? In agent Donald Maass' books on writing, he talks about all the work that goes into selling people on theories like this, and Roth just didn't do it for me while Collins absolutely did.
My WIP is the first in a trilogy, but it has a self-contained story. The two protagonists are very opposite girls on two paths that are set up from the beginning as two trains on a collision course. Both have their own (subtly intertwined) arcs that converge in the third act. In the first book, the girls have a coming of age story as the antagonists make careful plans to set up a civil war and each protagonist is unknowingly manipulated to be involved. (They come of age as they both start out on paths completely unlike them, then realize the reality of the world when they find out they've been duped by people they trusted.). The book ends with the girls' coming of age arcs being resolved, but the war starting anyway. So you have the self-contained arcs of the girls coming to a resolution, but the antagonists still succeed in setting off a war, which is where the next book picks up. Also, the girls' personal arcs are given a tinge of danger in the last two scenes (one is reunited with the love she's been looking for only to find out that the man who date raped her and slut-shamed her is her love's father, and the other is held captive by the boy she thought loved her who turned out to be an antagonist... so she is not in physical danger--as he has plans for her and still cares for her to some extent--but she has been betrayed.)