Which three years is the Pasuk referring to?
Rashi, Ramban #1 (reciting the Sifra) and Targum Yonasan (on Pasuk 23): It refers to the sixth, seventh and eighth years 1 of the Sh'mitah cycle. 2
Ramban #2 (citing a second opinion in the Sifra): It refers to the seventh, the Yovel and the year after the Yovel. 3
Rosh Hashanah, 13a: 'Don't read it "li'Shelosh", but "li'Shelish" ' - To teach us that crops all fall under the category of 'Tevu'ah' when they have reached a third of their growth. 4
Rashi: In fact, it refers to the sixth year, from Nisan until Rosh Hashanah and the entire seventh and eighth years. They will then plant in Marcheshvan of the eighth year and harvest in Nisan, gather it in from the fields in Tishrei of the ninth year and plant afresh in Marcheshvan of the ninth year - as the Pasuk goes on to explain.
And it is because the Torah refers to this as three years that the Gemara in Rosh Hashanah Darshens "li'Shelosh" as if it ha written 'li'Shelish' ? to teach us that corn falls under the category of Tevu'ah when it has reached a third of its growth. (regarding Ma'asros and other issues). See Torah Temimah, note 111.
According to Rashi (in Pasuk 22), when Sh'mitah is followed by Yovel, the land will yield four years produce in the sixth year, enough to last until the ninth year.
Regarding many issues, including BIkurim, the Omer and the SH'tei ha'Lechem. See Torah Temimah, note 111.
Why does the Torah add the (otherwise superfluous) word "Vetzivisi es Birchasi Lachem"?
