三年

Noun

 * 1) three years; the third year
 * 2)  the third school year
 * 3)  a long period of time, several years
 * 1)  a long period of time, several years
 * 1)  a long period of time, several years
 * 1)  a long period of time, several years
 * 1)  a long period of time, several years
 * 1)  a long period of time, several years

Usage notes
(middle school, junior high school) and (high school) each last three years in the contemporary.

Noun

 * 1) three years; triennial
 * , 伊勢物語 (), volume 24:
 * 1) three years old
 * 1) three years old

Usage notes
The reading みとせ is less common and may be more restricted in usage than さんねん.