From Chabad.org: This section addresses an obvious concern: “What will we eat in the seventh year if we do not sow our gather our grain?!” G‑d reassures us that He will bless the sixth year’s harvest, and it will produce enough to provide for three years! The Torah then gives the rationale for the prohibition against selling land for perpetuity (instead, land can only be “leased” until the Jubilee – 50th- year) — “Because the Land belongs to Me; you are strangers and residents with Me.” The seller of land, or his relative on his behalf, has the option of “redeeming” the land from the purchaser — provided that two years have past from the date of purchase.
Technically, if the sixth year harvest is needed to supply for the eighth year, because the eighth year harvest hasn’t grown yet, that means that the sixth year harvest will actually be used in the seventh year. That means that the sixth year harvest will be needed for the seventh (1) and eighth (2) years. Why then does it say that the sixth year harvest will last for three years? Rashi explains that it includes partial supply in the first year, since they will plant on Rosh Hashana and reap in Nissan of that year, so they’ll get six months of produce in the first year. The third year would also include that partial produce status.