Rashi on Leviticus 25:21/רש"י על ויקרא כ״ה:כ״א

1לשלש השנים [IT SHALL BRING FORTH INCREASE] FOR THE THREE YEARS — i. e. for a part of the sixth year — from Nisan till the New Year of the seventh — for the whole seventh year and for part of the eighth year, for in the eighth year they will sow in Marcheshvan but will harvest only in Nisan (so that they will have to depend on the harvest of the sixth year in the eighth too).