The Minchas Chinuch (mitzvah 365) poses the following question: The Rambam in Hilchos Sotah (1:3) rules that it is possible for a husband to warn his wife against secluding with two men at the same time. If she secludes herself with them, she will become forbidden until she drinks. Why is she forbidden? Since there are three people there (the woman and the two men), it should have a status of being a public domain, and she should be permitted!
Incidentally, it is evident from the Rambam that a woman can become a sotah even though she did not violate the laws of yichud (a woman being secluded with a man), for there is no yichud in a case of a woman and two men. This can also be proven from the halacha that a woman can become a sotah when she secludes herself with her father or brother, even thought here is no prohibition of yichud there as well.
Reb Avi Lebowitz brings another proof to this concept: The Gemora in Brochos (31b) states that Chanah threatened Hashem that if she doesn’t have a child, she will behave in a manner where Elkanah will warn her and she will then seclude herself with that man. After drinking the water and emerging innocent, she would be blessed with children according to Rabbi Akiva, who says that if she was barren, she will be blessed with a child. How could a righteous woman like Chanah put herself in a situation where she would be violating a prohibition of yichud? Clearly, it must be possible to accomplish a seclusion that would render a woman a sotah even without a yichud prohibition! (There are commentators who discuss various scenarios where a woman can be rendered a sotah from secluding herself with one man even without violating the issur of yichud.)