פירוש על בבא בתרא 56:3

And (the gemara) answers: What is the reason? - We say three years takes it out from the Chazaka of the first master (owner), we say: perhaps like he (the possessor) said. (Example:) this one says "I acquired it from you" and this one says "you stole it from me", and we don't know with who is the truth. Comes (the) chazaka of three years and it establishes it in(as) his (the possessor) property, and we say perhaps it truly is like he (the possessor) said (that he acquired it). But without an argument (of the possessor, such as "I aquired it"): from where did he aquire so as to eat for three years? (we can can ask what right has he to be there).