Or HaChaim on Leviticus 25:21/אור החיים על ויקרא כ״ה:כ״א

1 וצויתי את ברכתי, "And I shall command My blessing, etc." The blessing referred to in this verse does not pertain to the growth of the crop; that blessing has already been mentioned when the Torah spoke of the earth giving its full yield. The blessing the Torah speaks of here is in the nature of what we read in Kings I 17 and Kings II chapter 4 where the prophet made the little bit of oil of the respective petitioners stretch miraculously. This is alluded to in the words of our verse that ועשת את התבואה, i.e. that the existing crop will be made to last for three years. My comments on Genesis 33,9-11 are relevant to this verse.

2לשלש השנים, for the three years. The meaning of the verse is that the blessing will be in effect during the sixth, the seventh and eighth year. Statistically speaking the period of three years the Torah speaks of commences in the month of Sivan during the sixth year, i.e. the month most of the grain is harvested, and continues till the month of Sivan in the ninth year. The Torah had to write the word לשלש השנים so we would not think that part of a year is considered a whole year [as when we count the years a king reigned. Ed.]