This "third" is the ordinal number for three. Every three years the people of Israel gave a tenth of the harvest to the poor. (See: [[rc://en/ta/man/jit/translate-ordinal]])
Here "gates" means towns or cities. Alternate translation: "so that those within your towns may have enough food to eat" (See: [[rc://en/ta/man/jit/figs-synecdoche]])