Why davka three years? TB Baba Batra 28-29

 In massekhet Kiddushin we previously learned that a person acquires something using one of three methods, cash, documentation, and hazakah (חֲזָקָה), a proprietary act. Chapter 3 of our massekhet which begins on daf TB Baba Batra 28 provides a qualified additional understanding of what a hazakah is. “With regard to the presumptive ownership of houses; and of pits; and of ditches; and of caves, which are used to collect water; and of dovecotes; and of bathhouses; and of olive presses; and of irrigated fields, which must be watered by people; and of slaves; and all similar property that constantly, i.e., throughout the year, generates profits, their presumptive ownership is established by working and profiting from them for a duration of three years from day to day. If the one in possession of the property can prove that he worked and profited from it for the previous three full years, there is a presumption that it belongs to him, and would remain in his possession if another were to claim that the property belonged to him or to his ancestors.” (Sefaria.org translation)

Hazakah, presumptive ownership, requires three continuous years of occupancy or use otherwise the original owner can object and claim that the property still belongs to him. Presumptive ownership requires more than just three years of continuous occupancy. The hazakah has to include a valid claim (ta’anah-טענה) like I bought it, I inherited it, or it was gifted to me. 

The Gemara wants to know where did the Mishna come up with the number three. The Gemara first suggests that there is an analogy between the three years of presumptive ownership to the case of a forewarned ox (shor mu’ad- שׁוֹר הַמּוּעָד) that has gored separate three times. With the fourth goring, the owner of the ox has to pay full damages. The Gemara shows that the analogy is far from perfect. In fact, in the end, this analogy only works for Rabbi Yishmael who only requires three consecutive harvests and not three consecutive years. “But with regard to a field of trees, once he gathered his produce, and then harvested his olives, and then gathered his figs, these three harvests are the equivalent of three years. Rabbi Yishmael is of the opinion that three harvests are sufficient.” (Sefaria.org translation)

On daf TB Baba Batra 29 Rava provides the reason why the rabbis, who don’t accept the above analogy, require three years. “Rava said a different reason: A person is careful with his document detailing his purchase of land for the first year after the purchase, and he is also careful for two and three years. For more time than that, he is not careful and might discard the document if no one has lodged a protest concerning his possession of the land. Therefore, the Sages ruled that after three years have passed, he can prove his ownership by means of presumptive ownership.” (Sefaria.org translation)

You can imagine safekeeping documents back during Talmudic times was not easy. No matter what period of time a person lived, storage space for important documents is limited. Back in Talmudic days, a person was only expected to keep important documentation for three years. The person could either repurpose the material document was written upon or discarded it. Today the IRS only expects a person to keep his documentation for seven years.