1) THE REASON WHY A CHAZAKAH NEEDS THREE YEARS
OPINIONS: The Gemara states that in a case in which a person occupied a field for one year and then died, and his son inherited it and occupied it for two years, the combined years of occupancy combine to make a Chazakah of three years, and the field is established to be the property of the son.
The Rishonim and Acharonim discuss the nature of how a Chazakah of three years works, based on this Gemara.
(a) The RAMBAN questions whether the son must be an adult over the age of thirteen, or whether he can make a Chazakah even if he is a minor. He cites an opinion that maintains that only an adult son can make a Chazakah with the accumulated years of his occupancy and his father's occupancy, because if the son is a minor it is not possible for the original owner to submit an effective protest against him. Even if he would protest, it is not possible to require a minor to keep the Shtar that attests to the purchase (since the manner of minors is to lose things of value), and therefore the original owner does not bother to protest.
The Ramban disagrees with this view. He states that the reason why a Chazakah of three years is effective is not merely that the Machzik is not expected to keep the Shtar for three years. Rather, when a person occupies a field he essentially creates a Chazakah immediately, because the owner of a field does not allow a stranger to occupy his field even for one moment without objecting. However, for the first three years after the sale, the original owner is entitled to demand that the Machzik show his Shtar, and if the Machzik cannot show the Shtar this creates a weakness in his claim and suggests that the field does not belong to him. After three years, however, the fact that the Machzik does not possess the Shtar does not weaken his claim of ownership, because a genuine owner does not bother to keep his document of ownership for more than three years.
Why, then, according to the Ramban, does a minor not acquire a Chazakah immediately? Since, for a minor, the lack of a Shtar is not a flaw in his claim (for he is not expected to look after it), he should be able to make a Chazakah immediately! The Ramban answers that it is not logical that a minor should benefit as a result of not being mature enough to be responsible. The KEHILOS YAKOV (18:8) explains that since the true owner is entitled to wait up to three years to challenge an adult who occupies his field, the Chachamim did not want to institute a shorter time period in which the original owner must protest the occupancy of a minor (as such a Takanah would benefit the minor for being irresponsible).
(b) The KETZOS HA'CHOSHEN (140:2) gives a different reason for why the Chazakah of three years is effective. He explains that it is a Takanas Chachamim for the good of the world. Most purchasers do not keep their Shtaros for more than three years. They assume that since they have been living on the field for three years and no one has challenged their ownership, they no longer need to fear that anyone will challenge them. If the Halachah would require buyers to keep proof of purchase for more than three years, people would be very apprehensive about purchasing property, and this would have a detrimental effect on the economy. Therefore, the Chachamim instituted that a Machzik is believed to say that he purchased the property when he has a Chazakah of three years. If someone has a claim against the Machzik, he has the option of protesting within three years, since the Machzik must keep his Shtar for three years from the date of the protest.
In summary, according to the Ramban, the Chazakah of three years is effective because once three years have passed, his lack of possession of a Shtar is not considered a weakness in his argument. According to the Ketzos ha'Choshen, the Chazakah of three years is a Takanas Chachamim that a buyer does not need to keep his Shtar for more than three years, which was instituted for the benefit of buyers and sellers of land.
2) MAKING A CHAZAKAH ON ONE'S PARTNER'S SHARE OF THE FIELD
QUESTION: Shmuel states that when a field is owned jointly by two partners, one partner can make a Chazakah on the other's share by occupying it for three years (and claiming that he bought it). The RASHBAM (DH Aval) explains that he is believed with a Chazakah and a claim, because had his partner not sold him part of the property, they both certainly would have worked on the land together and taken the produce together.
The Gemara asks that there is an apparent contradiction between two statements of Shmuel. Ravina resolves the contradiction by saying that both statements of Shmuel refer to where the partner making the Chazakah is occupying all of the field. However, when the field is large enough to be divided, the partner's presence there establishes a Chazakah, because had he not owned the entire field, his friend would not have allowed him to occupy all of the field but would have insisted on dividing it. In contrast, when the field is not large enough to be divided, the partner's presence does not establish a Chazakah, because in such a case it is normal for one partner to use the whole field for three or four years consecutively even while the partnership is still intact.
The RAMBAM (Hilchos To'en v'Nit'an 13:8) rules that two conditions must be fulfilled in order for one of the partners to establish a Chazakah. First, the field must be large enough to be divided. Second, the partner must use all of the field.
The RASHBA cites RABEINU CHANANEL who says that even if only the first condition is met (the field is large enough to be divided), the partner is able to make a Chazakah on the field even without fulfilling the second condition (he does not use the entire field). Rabeinu Chananel maintains that if the other partner would not have sold the field previously, he would not have permitted his partner to use even part of the field for three years without protesting, since it is possible to divide it (and dividing it would have enabled each partner to claim sole ownership on his half.)
According to Rabeinu Chananel, who requires only the first condition to be fulfilled, why does Ravina state that Shmuel's ruling refers to when the partner occupied the entire field? Ravina should say that establishing a Chazakah depends merely on whether or not the field is large enough to be divided!
ANSWER: The BEIS YOSEF (CM 149:1) answers that Ravina says that the partner occupied all of the field because he wants to teach a Chidush. Even though the partner uses the entire field, one might have thought that this is sufficient proof that the field belongs to him. Therefore, Ravina teaches that the Chazakah does not depend on this factor at all, but rather the Chazakah depends on whether or not the field is large enough to be divided. If it is not large enough to be divided, then a partner's usage of the field is never able to establish a Chazakah, because the other partner would not object to his friend's use of the entire field, since he would not use the field simultaneously with his partner under such crowded conditions. (Y. MARCUS)