It's still possible for Kar, Kan, and Terezi to ascend - we don't know the contents of Derse and Prospit proper. The alpha kids ascended on their slabs in the destroyed moons (7738) but the main planets are still intact (prospit damaged, but intact.) And we know slabs can also sometimes in special cases be present in the main planets (5197), and of course that the session is omniscient, including about the arrival of extra players (6720).
The slabs at the center of the moons of Derse and Prospit were destroyed by the Condesce and Lord Jack and even if they were still in-tact, none of the living trolls have the Heart, Life, Void, or Hope aspect. The only Quest Bed that could be used by the trolls is Jade’s Space bed being used by Kanaya but those require a dream body and hers is dead.