MG1: The best one. Overall very, very good.
MG2: Very good, and some single moments that eclipsed some from MG1, but a disappointing final episode.
MG3: Excruciatingly painful acting from some of the adult actors, and a very slow build-up which I had issues with. But it got better, and it was worth watching, for certain episodes and individual performances. As a whole definitely the weakest of the three seasons, but it had its moments.
I agree with you. I loved the first one. Sequel is usually never better than the first. MG3 was a disappointment for me. The acting was very weak. The "gakuen" loses a lot when they fight against adults instead of each other. I was glad I watched it to the end though since Jurina was kicking ass again!