There is a difference of opinion as to whether this tithe in the third year is the same tithe as earlier in Deut 14, or a second tithe. Note the different opinions:
- Ellicott: (28) At the end of three years thou shalt bring forth ail the tithe.—This is called by the Jews Ma’aser ‘Âni, “the poor’s tithe.” They regard it as identical with the second tithe, which was ordinarily eaten by the owners at Jerusalem; but in every third and sixth year was bestowed upon the poor.
- Barnes: Compare the marginal references. The tithe thus directed in the third year to be dispensed in charity at home, was not paid in addition to that in other years bestowed on the sacred meals, but was substituted for it.
- Poole: At the end of three years, i.e. in the third year, as it is expressed, Deu 26:12. So, in the end of three years, or of seven years, is the same with in the third or seventh year, as appears by comparing Deu 31:10 Joshua 9:16,17 2 Kings 18:9,10 17:6. All the tithe of thine increase. I join with those expositors who make this the same tithe with the former, Deu 14:22, as being called by the same title without any distinction between them, save only as to the place of eating them. See Poole "Deu 14:22", and See Poole "Deu 12:17".
Regardless of which view one adopts, it was one of the many features of the Israelite economy that provided for the poor and homeless. The gleaning laws illustrated by the story of Ruth (Ruth 2) was another such provision.
The reason or function of the three year period is a mystery - why this should not have not been every year i can only surmise; however, it is like that it was to prevent entrenched laziness in poverty.
All the "poor" laws in Israel, such as the gleaning laws, the generous lending laws, land law and inheritance laws operated continuously. Therefore, I assume that every third year provided a small luxurious boost for the one month for the poor.