What are the three necessary elements of a rhetorical situation?

A rhetorical situation is the context of a rhetorical act, made up (at a minimum) of a rhetor (a speaker or writer), an issue (or exigence), a medium (such as a speech or a written text), and an audience.

Mainly, the exigence, which is the issue, the audience, and a set of constraints  (are parts of the situation because they have the power to constrain decision and action needed to modify the issue)