With the "common" type three speed bikes (not the rare or high-end stuff) the early Schwinn three speeds are fairly collectable. The ones made from 1938 through 1952 are usually the ones collectors want. The parts for those also have a premium (e.g., "Schwinn Built" brakes, early bars, razorback stems, Schwinn Built levers, old style shifters, etc.).
The ones made from 1953 through the mid-1960s aren't quite as strong with collectors, but still do well if you have a tall 23 inch frame model, or unusual colors, or really good and ornate graphics. They can make a good balance of affordability and classic features.
The late 1960s through the end of the run are kind of marginal with collectors, unless you have a pristine 23 inch frame bike. They're perfectly functional bikes, but don't have the draw for collectors associated with the earlier stuff. Some of these later bikes still turn up at yard sales for $20 or the like.
Throughout the run, short frame is a deduction, standard frame is middle, and there's a premium for the tall frame. Most adult males will want the standard or the tall frame if you're going to ride.
The funny thing I've noticed is that there is interestingly little cross-over with people who collect English three speeds. Some of those collectors kind of look down on the 1950s-60s Schwinn three speeds because of the welded frames, flat blade forks, vinyl saddles and the like. I collect both, and you get something a little different riding a Schwinn versus a Raleigh or something of that sort. If you're really into old three speed bikes, you should have at least one of each.