The place for nice, old 3-speeds is Craigslist in your local area.
A bicycle-computer mounts on your handlebars and takes readings from a magnet & transmitter mounted on the fork - usually. They are connected either by wire to the computer on the handlebars, or they are wireless and send data through radio-frequency from the transmitter. They measure your current speed, maximum speed, average speed, distance, time so forth. This is displayed on a screen on the computer. Here are some examples:
http://www.cateye.com/en/product_listing/51