Date: 9 Teves 5569
From: Rabbi Moshe Sofer
To: Rabbi Tzvi Avigdor
The Chasam Sofer resolves the apparent contradiction between the Rambam's ruling that a wife drinks Sotah waters when secluded with two warned-about men simultaneously and the principle that doubtful impurity in a public domain (three persons) is ruled pure. He argues that only men whose seclusion with the woman is permitted count toward constituting a public domain; since each warned-about man is analogous to a contaminating sheretz and his yichud is prohibited, none of them count, leaving the situation classified as reshus ha-yachid regardless of how many warned men are present.
Source: Shu"t Chasam Sofer EH II §102