(Mishnah): If two witnesses testify that he ate (the produce of the field) for three years and they are found to be false witnesses, they must pay the full value of the field to the owner.

If there were two for the first year (of chazakah), another two for the second year and another two for the third year, the three groups divide up the payment.

If each year had testimony from a different brother but the other witness of each year was the same person, they are considered to be both three testimonies and one testimony.

(Gemara): If the owner asked him, "What are you doing in my field", and he replied, "As I have been here for the years of chazakah". If he brought those witnesses but the owner then brought witnesses to prove them false, he must return the field but the produce of the last three years may be eaten by the resident (as the witnesses are only disqualified from this point and onwards, but not retroactively.

R. Zeira: A false witness is only invalid in Beis Din (meaning, again, that he is only disqualified from this point and onwards).