I did a very similar trip this past summer but slightly longer (GVA for 2 nights, Montreux for 3 nights, Zermatt for 5 nights). Whether Zermatt is as enjoyable as you hope is really going to come down to weather. We got lucky and had only 1 really rainy day the entire time we were there. Montreux is gorgeous but I also found it to be a bit of a sleepy town. I wouldn't have minded more time in Geneva, but it was definitely the right place to start out and get the jetlag done with.