Right now, I'd say their chances of meeting my expectations are pretty good since they haven't even officially announced anything beyond that they are working on a "dark fantasy game." CDPR also seems to do its own thing instead of mimicking other games like the developers that work for big publishers are forced to do, so that helps. But, I've also been burned by companies because I believed the hype (Bioware), so I'll try to manage my excitement when we get details. So far, though, CDPR hasn't given me any reason to doubt them, so I guess I'm optimistic.
The thing is, is that a third entry in a trilogy is always tricky. You have a successful first entry. The second introduces new characters and themes and is sometimes bigger and better than the first. The third is where things can easily go wrong. You can take what made the first two entries successful and add to them, but do you risk trying something new? It can and has worked before, but you also don't want to overdo it, because you could....over-saturate it if you will with so much stuff you end up forgetting some of the more important bits like story and character development. You want to try and balance all of that while adding some new elements. But you don't really want to go too big with those elements, because as I said, you could overshadow what made the first two entries so great in the first place. This applies to both movies and games. The more entries into a specific series like The Witcher or Mass Effect, the greater the chances of the franchise wearing itself out or overstaying its welcome. It doesn't always end up that way, but the risks are always greater with a third game/movie. Just something to think about.
The majority of the thirds parts are made by contractual obligation to raise more profits, the big distributors decide whether to create or not the third party. The case is significantly different with CDPR and TW3: its creators who decide to create it with enthusiasm and desire to excel. For now.