Honestly, it being a trilogy is probably a good way to get an actor like Pattinson on board. Much better chance he'll sign on for a 3 movie deal, than sign on for 8 without a clue of what the creative direction would be that far down the road. Realistically you don't go into a project like this expecting it to be a one-off either, so there has to be some consideration for what the overall commitment is from a business standpoint. If the movie turns out to not be successful, then you reevaluate.
I like trilogies personally. When they're done well, it's one of the most satisfying forms of cinematic storytelling. Maybe I'm just old fashioned that way, but give me a great trilogy over an elaborate interconnected universe any day of the week. Mind you, that's also not to say that Reeves couldn't potentially do something like with his Apes trilogy where he concludes a 3 story arc, but the door is left open for more.
I do agree that we're getting ahead of ourselves though and we first need to see what this movie even is before worrying about parts 2 and 3. Cart before the horse and all that.