2nd will be your most efficient and durable "Cruising" gear....
Yeah, if Ichi is still making those cogs, safest bet is an AWC or an S-RC3 with one of those. (Yeah, you can run Shimano but....well, that's a topic for a different post.) With all the 3 speeds, the 2nd gear is direct drive, so gear it like you would your singlespeed, and enjoy an underdrive for the hills and standing starts, as well as an overdrive for downhills, sprints, and whatever "racing" you involve yourself in on your ol' cruiser.
If you don't have the frame yet, get yerself a Schwinn MW that came with gears... that'll buy you another 6mm in the dropouts. Spread it the other 4mm, put the wheel in, and go! If you're switching to a 559 wheel, you'll need to either run smaller tires (1.75") or no fenders, or you gotta get creative to fit balloons with fenders....
It's also more efficient and durable than the under/overdrive gears.... so, theoretically, that's the one you want to be in the most. I just bought a Sturmey 3 speed for a tadpole cargo trike I'm looking to get; I'm gearing it a 52" in direct drive, b/c I'll want as low as possible for any loads I'm hauling. I could see spending as much time-- or even more!-- in 1st than I will in second, but that's an extreme case. (If I had to run the trike singlespeed, I would have gone with 45" or so...)But, if I'm setting up a bike for more typical riding, I'm going to run something like a 44x18 or 63.5" in direct, which'd give me a very usable unerdrive gear for climbs, and a considerably bigger gear for riding fast. Of course, since the ratios change by percentage rather than fixed increments, the jumps between gears become larger as the direct drive gear gets higher.... but it's still a pretty good rule of thumb for a starting point.
