Rabbi Baruch Bodenheim: Monday Night Gemara Shiur Bava Metzia 27A Tanu Rabbanan

1) Gemara from 2 dots “Tanu Rabanan .. until “Rava amar 2) Question; The gemara listed three scenarios which are  excluded from the obligation to return:  1. A lost item that is worth less then a Pruta, 2. Aveida of an Akum is 3. Item lost by banks of river.  Is it also permitted to keep these items? & if it is, Why is in not considered stealing? What removed the ownership of the original owner?