There is no command for tithes to be brought once every three days. Under the old covenant, Israel was required to give a tenth of each year’s agricultural produce and livestock increases to support the ministry of the priests at the temple and elsewhere. It seems to be as if Amos is suggesting that the people of Israel were tithing more than what was required of them.
4 “Come to Bethel, and transgress; to Gilgal, and multiply transgression; bring your sacrifices every morning, your tithes every three days;