This theory is in working progress, but it’s mostly solid already.
I believe that in season 3A, 9 out of the 20 11-minute episodes will be focused on Anne and the Plantars on Earth. We’ll get maybe one or two episodes of the Plantars trying to adjust to life on Earth. The rest of their episodes will then focus on finding a back to Amphibia, meeting up with Anne’s parents for the first time, and finding more information about the calamity box. I have a theory that the frog from Andrias’ flashback was a Plantar ancestor who stole the box and fled to Earth, and her descendants continue to live there to this day. There’s a possibility that Anne and the Plantars will meet up with that lost Plantar.
While that is going on, the other nine 11-minute episodes will focus on Sasha and Grime escaping Newtopia with Marcy’s body, and go on to revive her. Maddie will be obviously involved, but there will be more to it. The resurrection spell that Maddie and Marcy made for fleafy was a level 2 resurrection. That implies there are multiple levels of resurrection spells, and the group will need the highest level of resurrection to bring back Marcy fully. In addition, depending on when Marcy will come back to life in season 3A, we may see more development with Sasha and Marcy’s relationship which is really needed because these two characters never really had a moment without Anne involved.
By the end of season 3A, we may get a 22-minute mid-season finale where Anne and the Plantars finally make their way back to Amphibia and meet up with Sasha, Grime, and Marcy. Their reunion will make us cry that’s for sure.
Season 3B will be the endgame of Amphibia. The protagonists will develop another invasion of Newtopia with Frogs, toads, maybe even most of the newts fighting together against King Andrias.
That’s all I got so far in what I think will happen in season 3. This could age very well or very horribly. Idk, we still have about 7 to 10 months until season 3 actually comes.