As I said before, the first season was to present BoJack being a complete jerk and ended with him realizing that. Until then he never noticed what a toxic person he was. Season 1 BoJack is a terrible person who doesn't even realize how terrible he is.
The second season was him dealing his this new self-image, accepting it and finally deciding to change at the season finale. Season 2 BoJack is someone who is suddenly aware of his flaws and is completely overwhelmed by this realization.
The third season has BoJack trying his best to change, he is trying to be nice to Todd (even though it's never his first reaction), be there for Princess Carolyn, trying to get close to Diane again... Season 3 BoJack is more optimistic than season 2 because he is actively trying to be a better person and you can see the results as his relationship with all those characters seem to improve and at the same time he is getting some recognition for "his" work.
The focus of season 3, however, is that wanting to be a good person is not enough, you don't suddenly become a better person just because you are trying when you are inherently broken. In the end, BoJack fails. Despite trying to treat Todd with respect, he still doesn't give a fuck about his feelings and have sex with Emily even though he thought Todd had feelings for her. Despite trying to do the best for P.C., when their interests conflict he instantly dismisses her and condemn her to bankruptcy even though his career only exists until this day thanks to her. When he thinks the world is finally in love with him, he feels like he can treat Diane as shit because he suddenly is adored by many and she is just one in many now. At the end of the season, as Diane foresaw, there was no one to stop BoJack when he decided to kill himself.
Season 1 is BoJack suddenly realizing who he is. Season 2 is BoJack figuring out what do with this realization. Season 3 is BoJack doing something about it but failing hard.
In a sense, season 3 was the opposite of 2. The latter started with BoJack being low, spiraling down and then starting to improve. The former started with him high, going to the highest and then falling to the lowest he ever was.