That would be fine if the storylines were wrapped up at the end of season three but shows not only fail to do this, they often end on a cliffhanger and then the series gets cancelled. I don't know about other people, but I wait until a show has been on for years before bothering with checking them out. There are a few that I knew were continued as graphic novels, which I thought was a decent compromise but if I know there's an unresolved cliff hanger and a bunch of half finished storylines, I don't bother starting the show.
A three year series is fine but treat it like that in the third season before too many people's behaviors change and it starts to become difficult to get their attention to new series because they fear it ending suddenly without resolution.
I don't mean to sound like some "well akshuallyyy"ing Comic Book Guy figure, but you must realise it's not real life? If it's a Poirot-type mystery where the entire payoff lies in the resolution, then I totally understand, but for a regular TV show I can't see why you would enjoy the episodes any less simply because it might end without resolving all your questions. There's no real suspense - it's not real people, it's just a bunch of people performing in front of a camera. Being distressed about 'what happened to them?' seems, well, puzzling to me.
A three year series is fine but treat it like that in the third season before too many people's behaviors change and it starts to become difficult to get their attention to new series because they fear it ending suddenly without resolution.