I felt the same way too!
Nope, you're probably in the majority. Because manga that have a good happy ending for a threesome usually get thumbs down by the majority. I find that weird but it is the unfortunate truth. I, as you probably know, am in the minority.
It's not that I think a happy ending of a threesome is bad. In this case, I just felt like it didn't make sense for the story to come to that conclusion. Maybe this is my misunderstanding of the characters' feelings, but I couldn't follow the development of their relationships or their personalities.
Am I the only one who feels like the conclusion to the story is waaay too convenient and doesn't make any sense? It feels like the mangaka just gave up... （￣へ￣）