Pots & rheostats
Rheostats ALWAYS have only two connections since they're current-adjusting devices and go in series with a load. As mentioned, they are/were usually wire-wound units with high power ratings and have mostly been replaced with solid-state controls, e.g., a triac lamp dimmer. Old theater lighting and motor speed controls used to use huge, heat-producing rheostats.
Potentiometers (the name kind of gives it away here) ALWAYS have three connections since they're voltage dividers. Pots were (and somewhat still are) commonly used as the volume control in a radio where signal was fed to one stationary terminal, ground to the other and the variable audio level was picked off the wiper and ground.
If you look through today's industrial electronics catalogs, you won't find much in the way of rheostats. Since the only thing missing from a rheostat is a terminal (the insides are all there), they usually make them as high-power potentiometers and let the end user ignore one terminal if he needs it as a rheostat. That way, they only need to make and stock one part number vs. two for each value and power rating.
Dean