Right, RD; RCA is the plug on the end, not the wire itself, although it seems that the term "RCA cable" is fairly frequently used for a cable that isn't of 75ohm impedance, while one that is 75 ohms is termed "digital", "video", etc. The usual clue if the red and white color coded cables are different from the yellow in a three cable set is if they're obviously thinner. Some of the sets, as Ken mentioned, have three identical cables for both the audio and video connections.
As JohnB. mentioned, if three red/whites are less expensive than two yellow/red/whites, you can go with that. No particular impedance is required for analog audio cables.
