There's nothing wrong with taking your vintage 3-speed out for an excursion. I've done it. Not a century, but a good long day trip or two. A couple of metrics and a few shorter rides. You cause me to think about a full 100 miles. Those bikes are the most comfortable rides I know of. The one in this link inspired me to convert an old rigid mtb and a tour bike into a sort of 3-speed on steroids. That is, same handlebars ( but 2" wider ) and Brooks saddles. It's a 1964 Mercury Courier that I cleaned up and now it just won't get rid of me.
http://i5.tinypic.com/2it68pj.jpg