That's an excellent question. The only other example I can think of would be the first three Bourne films, or possibly Krzysztof Kieślowski's Three Colours trilogy. With the Toy Story films, I'd say each was equally brilliant, so that doesn't count.

I suppose you might argue that the first three James Bond films do that, but I'm always torn on which is better between From Russia with Love and Goldfinger. At present, I'm inclined towards the former. :)