Q. Can one say it (Shalom Aleichem at Kidush Levana) to one person or it needs to be said to three different people?

A. The most established tradition in Ashkenazi communities is to address three different people. However, from the text of the Remah (O.H. 426) it would seem that he is greeting only one repeatedly. So too, it is mentioned, was the custom of the Brisker Rov (Mipniney Horav p.94)
Shalmei Yosef (p. 69) mentions that an established custom by Sephardic and Yemenite communities is that all congregants repeated together three times addressing everyone present and that is why it is said in plural. He also relates the tradition of turning right, then left and finally facing front when addressing all present.
Rivavos Efraim (5: 300: 2) mentions that the tradition of the Staipler Gaon was also to address three different people while turning right and then left.
As mentioned in the previous question, Horav Shlomo Miller’s Shlit”a opinion is that a fundamental basis for greeting Shalom Aleichem is the receiving of the brocho of Sholom as an answer. As in Hatavat Chalom (the act done to mend and soothe bad dreams) a threefold blessing is accustomed and three different people are required, since the brocho of one individual may not be as accepted and effective as his peer’s blessing.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit”a