Most days, I commute just a few miles on fairly flat terrain. I also have many 1/2 mile rides to the grocery store, park, pharmacy, etc with my kid.
I always want to downshift at stop signs and stop lights, and it is nice to be able to get into a high gear when you want to sprint for that yellow light, or go fast downhill, or keep up with that guy on the fixie.

You could definitely ride a single-speed bike for all you needs, like millions of people in Europe and around the world, assuming there are no huge hills nearby. But I think you may have more fun and find your bike more useful if you can downshift for headwinds and at stoplights. So if the price is not an issue, get the 3-speed.