Leviticus 25:21 - Joseph Benson’s Commentary on the Old and New Testaments

For three years Not completely, but in great part; namely, for that part of the sixth year which was between the beginning of the harvest and the beginning of the seventh year, for the whole seventh year, and for that part of the eighth year which was before the harvest, which reached almost until the beginning of the ninth year. This is added to show the equity of this command. As God would hereby try their faith and obedience, so he gave them an evident proof of his own exact providence and tender care over them in making provisions suitable to their necessities.