I'm a little confused about subdividing the WiFi radio band into 11 channels in the USA, 14 in Japan, and 13 in Europe, and to then use just 3 of them because of the overlapping problem. Why didn't they subdivide the band into 3 channels?
IEEE 802.11 (Wi-Fi) uses the 2.4 GHz ISM band that existed decades before IEEE 802.11 was defined, and the band was divided into 5 MHz channels.
Wi-Fi uses 22 MHZ bandwidth, more bandwidth than a single 5 MHz channel in the ISM band provided. The required Wi-Fi bandwidth does not map directly to a single channel and overlaps multiple channels, but Wi-Fi devices can pick any of the ISM channels as the center of the bandwidth it uses, overlapping on channels either side of the center channel chosen. It boils down to three center channels that do not overlap for Wi-Fi.
The other answer is correct. However, just to add on regarding the difference in availability of channels 12, 13 and 14 in the US, Japan and Europe/rest-of-the-world, it is because of differences in the regulatory situation (spectral occupancy is different in each country, and regulators want to avoid interference or excessive interference between bands).
- channel 14 is forbidden (by the FCC) for use in the US, whereas channel 12 and 13 are not forbidden but need to be used with lower power; hence generally, only 1 to 11 are used in the US, and within this bandwidth, 3 non-interfering channels can be selected
- channel 12 and 13 are used in most countries (including those in Europe) and Japan
- channel 14 is valid in Japan only, and only for DSSS and CCK modes of 802.11b