ok maybe my memory was not so good

Groot didn't learn more words, what he said was still I am Groot, but for the viewers it sounded as normal words. This is to indicate that we, as viewers, started to understand what Groot is saying. Same way Gamora understood him at the end and how the team understands him. It signifies us viewers becoming part of the family.

Also, 2nd movie was bad. It was very cartoonish and dumb. 3rd was an improvement, I have to admit. It was not bad in general. The movie still has multiple weak points and forced drama, but overall it was quite enjoyable and entertaining. I think whoever is hoping for Gunn's success in DC will be mildly disappointed. He is not their savior and despite him being above an average storyteller, his products are still problematic and far from being really good. It's usual for DC to reboot themselves every 6-8 years, so I bet on that instead of the huge makeover that will last long.