三年

三 (san, “three”) +‎ 年 (-nen, “year”)

• On’yomi: Goon
• (Tokyo) さんねん [sàńnéń] (Heiban – [0])
• IPA^(key): [sã̠nːẽ̞ɴ]

三(さん)年(ねん) • (sannen) 

1. three years; the third year

   三年(さんねん)ぶりに回(まわ)る。 ― Sannen buri ni mawaru. ― [It] has come around after three years.

2. (education) the third school year

   三年(さんねん)生(せい) ― sannen-sei ― a third-year student

   Coordinate terms: 一年, 五年, 二年, 四年, 六年

   • 2015 March 10 [2014/12/10], Shiozaki, Yūji, “第百五拾九話 [Chapter 159]”, in 一騎当千 [One vs. Thousands], volume 23 (fiction), 2 edition, Tokyo: Wani Books, page 130:

     大和(やまと)学院三年　新(しん)免(めん)武(たけ)蔵(ぞう)

     Yamato Gakuin sannen Shinmen Takezō

     Yamato Academy senior, Shinmen Takezō

3. (figurative) a long period of time, several years

Lower secondary school (middle school, junior high school) and secondary school (high school) each last three years in the contemporary Japanese educational system.

三(み) (mi, “three”) + 年(とせ) (-tose, “year”)

• Kun’yomi
• (Tokyo) みとせ [mìtósé] (Heiban – [0])
• IPA^(key): [mʲito̞se̞]

三(み)年(とせ) • (mitose) 

1. three years; triennial
   • a. 10th century, 伊勢物語 (The Tales of Ise), volume 24:

     あらたまの年(とし)の三(み)とせを待(ま)ちわびてただ

     Aratama no toshi no mitose o machiwabiteta da

     We have been waiting impatiently for year three to arrive

2. three years old

   Synonym: 三歳 (sansai)

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

• 1988, 国語大辞典（新装版） (Kokugo Dai Jiten, Revised Edition) (in Japanese), Tōkyō: Shogakukan “さんねん 【三年】”
• 1988, 国語大辞典（新装版） (Kokugo Dai Jiten, Revised Edition) (in Japanese), Tōkyō: Shogakukan “みとせ 【三歳・三年】”