Three, why not four? TB Kiddushin 3

Whenever a Mishna enumerates a specific number, the Gemara wants to know what’s excluded. The first Mishna teaches “A woman is acquired by, i.e., becomes betrothed to, a man to be his wife in three ways…She is acquired through money, through a document, and through sexual intercourse.” (Sefaria.org translation) Today’s daf TB Kiddushin 3 wants to know what is our Mishna excluding. “The Gemara answers: The number serves to exclude acquisition through symbolic exchange (khalifin- חֲלִיפִין, ) i.e., a pro forma act of acquisition effecting the transfer of ownership of an article. Although a woman can be betrothed by means of money or an item of monetary value, she is not betrothed if she is given an item by symbolic exchange.” (Sefaria.org translation)

Rashi explains why this symbolic exchange is unacceptable. Since the symbolic exchange can be worth less than a prutah, no self-respecting woman would agree to any form of symbolic exchange in order to enter into the first stage of marriage, kedushin. Rashi’s grandson Rebbeinu Tam raises problems with his grandfather’s explanation. What happens if the woman is willing to accept a symbolic exchange that is less than a prutah? The rishonim come up with two basic answers to Rebbein Tam’s question. Some teach that even though one specific woman may be willing to accept a symbolic exchange that is less than a prutah, her example is not strong enough precedent to accept symbolic exchanges. Others explain that since a symbolic exchange can be less than a prutah, it is excluded completely from the category of money.