That's why I said if MOS ends up being a successful enough to create more films in the future.
Also, with a trilogy, it'd be easier to tell a more cohesive story with a good ending in mind...beginning, middle, and end; just like how any well thought out trilogies have been written. And honestly, most of the trilogies that aren't so great, well I'd argue that it's because the creators didn't intend on stopping at the third film and it was only because of the disappointment results in the third film's earnings that they had to stop.
Plus, doing more films past the third film, without a endgame plan just gives more chances on screwing up somewhere down the line.