I feel like anyone saying 'there is a third?' Hasn't followed anything about Dune -both book or movie. If you know the books, you know Paul has a story arc that continues past the first book. If you were following the movie, Denis Villeneuve was loud and clear when making the first movie that he wanted it to be a trilogy that followed the main story arc of Paul (through Messiah. Not counting the preacher Paul of Children).

Are the movies worth it? Fuck yeah. Is this the last movie? Denis Villeneuve was clear about not wanting to adapt beyond the second book. So if it did happen, it wouldn't be with him, and he is the real talent and driver behind these movies. So most likely this is the last movie. It was never going to be some Marvel 30 movie slop fest or Avatar shoot-plot-from-the-hip saga.