I will be in Lauterbrunnen in September for 3 days. I hope to hike. What if it is raining (hard)?
It is raining hard, then do plan B or C
September is a wonderful time to visit. But, with weather in the Alps, you take your chances.
We hiked in the alps on two different trips. Both times we had fabulous weather, and both times the weather was horrible the day we left. My advice is to give yourself a couple or more days and hope you're lucky on at least some of those days. Have a plan B and be ready at all times for your plan A! We rented an apartment in someone's house while in the alps and I asked him what the weather report was for the day. He just laughed! He said, "There is no weather report, your in the alps. It can be anything from warm to snowing."
It is not common to have 3 straight days full of heavy rain. You are doing the right thing: sparing more days to deal with probabilities of better weather on at least one of them. If it is raining hard all time, head to some other city like Bern, Basel, Zurich for a day trip
I second Andre's recommendation. Consider Thun as well, which is really close and has an interesting castle right smack in the middle of the town. I know Interlaken takes a bad rap on this website, but you could find far worse places to while away a few hours. BTW, September is usually a great month in the Alps for hiking. You picked the time of year with the lowest chances of rain, although as the others note, it could still happen.