Season 3 had a lot of problems. I think the most damning thing I can say about it is it felt like the show lost it's "heart" in most of the season, there were more than a few episodes where I felt like I could be watching any other show. It didn't feel unique anymore. Not all of them were like that, there were still great episodes like Skooled and the whole mini-arc with Star transforming in her sleep, but it's very disappointing that the whole season wasn't more like those. Even most Season 2's shittier episodes still held the show's identity.
All that said it could have been much worse and the show hasn't been "ruined forever," the show's soul is still there and a lot of the bullshit that buried it this season seems to have been wrapped up by the finale. It still has time to turn itself around.