Which of these places (that we haven't already seen) will we see in season 3? Which places will we revisit?
Pigeon kingdom is basically just a joke, so I doubt Star and Marco will go there as anything besides a quick gag like the cats-with-human-faces dimension.
We'll definitely see Cloud Kingdom, since it was in the theme song. Maybe we'll see the Waterfolk kingdom, especially if the Mermaid Jackie theory doesn't turn out to just be a meme.
Since we got Tom episodes, we'll revisit the Lucitor Kingdom, and I don't think we've seen the last of Ludo's clan, so Star might visit Castle Avarius II.