So i read one of Matt Barly answers on Reddit AMA and yes, they finished production for just 3 seasons. He said 3 seasons is more than enough for him and he's agree with it. And it's kinda bummer that Disney still suffered to renew their shows up to 4 seasons. But I guess they never change.
So I expect Amphibia should have a proper good ending like the rest of the show. Star vs Evil ending was kinda disappointing. So I know Matt and the crew won't let us down. We believe in them.