Speaking for only myself (who else?), I'd say the third entry in a trilogy represents the point where enough is enough. I enjoyed the first one, so I wanted more. The second one gave me more, hopefully with a twist here or there. By the third one, I'm ready to move on. Been there. Done that. This, especially, with regard to gaming.
With film, I think the second entry tends to function as the second act, and the second act is where conflict arises. Good guys lose. Bad guys triumph. Things turn complex and interesting. This is where most stories are most compelling. Thing is: the vast, vast majority of dramatic works chicken the fuck out in the third act, giving "fans" what they think they want - a quaint, happy, *boring* ending where everything works out for the good guys. I'm not saying good endings are impossible, but I do believe you only end up with a "happy ending" if you refuse to finish the story. For the most part, I enjoy a more nuanced conclusion.
With film, I think the second entry tends to function as the second act, and the second act is where conflict arises. Good guys lose. Bad guys triumph. Things turn complex and interesting. This is where most stories are most compelling. Thing is: the vast, vast majority of dramatic works chicken the fuck out in the third act, giving "fans" what they think they want - a quaint, happy, *boring* ending where everything works out for the good guys. I'm not saying good endings are impossible, but I do believe you only end up with a "happy ending" if you refuse to finish the story. For the most part, I enjoy a more nuanced conclusion.