Trilogies that got better with each movie
I can't think of one besides the Captain America trilogy, since Civil War was the best and First Avenger, although a blast, was the weakest in the trilogy. Other than that, I can't think of any movie trilogy where each installment was better than the previous one.
Most great trilogies have a stellar first installment, followed by an even better second installment, concluded with a third installment that's good but the weakest of the three. The Dark Knight, Star Wars, Spider-Man, X-Men and Godfather trilogies all followed this trend.
The Toy Story trilogy had 3 equally good movies with maybe a slightly superior third movie. LOTR was 3>1>2. Iron Man was 1>3>2. Matrix was 1>2>3. However no trilogy comes to mind besides Captain America that was 3>2>1.