Originally all three formed a single tractate called Nezikin (torts or injuries), each Bava being a Part or subdivision.
He has to keep it as shomer hinam (watching over another's property without receiving any remuneration) until he can restore it to the rightful owner (Deut 22:1–3).
The laws as to what constitutes finding, what to do with the things found, how to guard against false claimants, how to take care of the property found, under what conditions the finder of a thing is bound to take care of it, and under what conditions he is not so obligated—all this is explained in the first two chapters.
The example given in the Mishnah of shomer sakhar is that of an artisan who undertakes to produce certain work out of a given material.
9:11-12, taking up again the subject of hiring, regulate the various terms for paying the due wages (based on Lev 19:13 and Deut 24:14–15).
The final chapter (10) regulates the relations between joint owners and neighbors, in dwellings and in fields.
The last case mentioned is especially interesting as showing a highly developed state of agricultural jurisdiction in the Mishnaic days.
[1] According to Rashi, the rabbis of Jerusalem were not of a contentious disposition, and settled difficulties without much discussion (compare p. 38b: "Are you from Pumbedita, where they make an elephant pass through the eye of a needle?