Jamieson, Fausset and Brown on Deuteronomy 14:28-29

28-29. At the end of three years ... the Levite ... shall come, &c.--The Levites having no inheritance like the other tribes, the Israelites were not to forget them, but honestly to tithe their increase [Nu 18:24]. Besides the tenth of all the land produce, they had forty-eight cities, with the surrounding grounds [Nu 35:7], "the best of the land," and a certain proportion of the sacrifices as their allotted perquisites. They had, therefore, if not an affluent, yet a comfortable and independent, fund for their support.