Hashem promises that the produce from the sixth year will feed the nation for three full years. This position subdivides regarding the situation spoken of in the verses:
Shemittah Followed by Yovel
The verses speak of the Jubilee Year. In such a case, there are two consecutive years in which sowing is prohibited, and so Hashem promises that the food planted in the sixth year will sustain the people for three entire years (36 months).
"וְעָשָׂת אֶת הַתְּבוּאָה לִשְׁלֹשׁ הַשָּׁנִים" – This position is able to understand the verse simply, that in the sixth year enough produce will be harvested to nourish the people for three years.1
"עַד הַשָּׁנָה הַתְּשִׁיעִת... תֹּאכְלוּ יָשָׁן" – According to this position, this verse means that the old grain will be eaten until and within (עד ועד בכלל) the ninth year.2
Context – This reading is supported by the location of these verses after the discussion of the Jubilee year, rather than after the laws of Shemittah.
"מַה נֹּאכַל בַּשָּׁנָה הַשְּׁבִיעִת"
Exceptional Case
"וּזְרַעְתֶּם אֵת הַשָּׁנָה הַשְּׁמִינִת"
Every Shemittah
The Shemittah year begins in Nissan, during the harvest season. As such, during every Shemittah cycle, no food is sown already in the second half of the sixth year, requiring the fifth year to make enough food to last for the entire sixth, seventh and eighth years.
Hashem blesses the nation that the crops planted in the beginning of the sixth year will suffice to nourish them across parts of three years of the shemittah cycle: during the second half of the sixth year, the entire seventh year, and the first half of the eighth year.