I have commuted on a three speed at various times and found it to be an excellent city bike for all the reasons mentioned above.

If you're concerned about performance on climbs, the best thing to do is give one a test ride on one of the climbs that you'll have to do on your commute and see how it goes. And remember that you may be able to swap out the cog on the rear hub for a bigger one that will give you a lower low gear for easier climbing...