Directly following the instruction concerning second tithe is the command for third tithe. This tithe is also not given to the Levites to do the work, but stored "within your gates." Nor is it to be set aside yearly to keep the Feasts, for this tithe is set aside "every third year."
The purpose for this third tithe is found in verse 29: It is God's welfare plan for the fatherless, widows, unemployed and the truly needy. After faithfully completing our third tithe year, we are entitled to ask God for a blessing upon His people (Deuteronomy 26:12-15).
Third tithe is paid in the third and sixth years of a seven-year cycle. God gives us the seventh year as a year of release, or sabbatical, then we begin another seven-year cycle in the eighth year.
John O. Reid (1930-2016)
Tithing: God's Financial System