Denis Villeneuve is on record wanting to do a trilogy and include Dune Messiah in Part 3. Assuming that Part 2 is as good as Part 1, then it would be safe to assume that there is enough public interest and financial support for 3 Dune movies.
Honestly whether Part 2 will be as good as Part 1 isn't that relevant so much as Part 1 wasn't exactly a massive box office success ($400M worldwide against a $165M budget) to begin with.
Villeneuve is already incredibly lucky he gets to make a 'Dune Part 2' at all, considering how cash-strapped WB correctly is esp. after the Discovery merger and David Zaslav's infamous stinginess in how he runs the business.
That is to say, quality notwithstanding, if Part 2 doesn't at least break even when it finishes its theatrical run, chances are slim there would be further Dune films from either Villeneuve or a less adventurous WB (just look at all the announced revivals from Harry Potter to Lord of the Rings).