I've never done a century on a 3-speed. I've ridden that over a couple or more days, but not all in one ride. I've become a true believer in shorter rides on a more frequent basis. I like to set aside maybe 60 - 90 minutes or so each day to ride at a brisk but comfortable pace (weather permitting) rather than try to do really long rides in one sitting. If it becomes uncomfortable due to pace, or weather, or injury, or whatever, then I stop as needed. I find that if you reach the point of "Something is wrong but I'm not stopping", that's when really bad things like injuries tend to happen. I also like frequenting areas where there are other bicycles on the roadways with some frequency because in those areas at least some of the motorists tend to be less surprised to come across a bike (a few never learn though...).

I guess there are people who do 100+ miles on a 3-speed and are used to very long rides. I would tend to think the terrain needs to be reasonable, given the jump between gears. I'd want at least one "bail out" climber gear in low or bottom gear, so probably a 22 tooth cog (or more if you like it really low). All the better if you have an S5-equipped bike you like and want to ride that. The ultra low and ultra high add a little more versatility on hilly roads. Make sure that saddle is well broken in.