Body formations of therapist and couple during therapy sessions mainly function to signal their degree of readiness to interact or their degree of engagement in the therapeutic process, which is one contextual display of their affective communication. For this study, we developed the Body Formation Coding System (BFCS), a 4-category instrument to assess engagement at the triadic level. This article presents the BFSC method as well as a first validation on a sample of 14 triads. The results show that (a) triads vary according to their degree of triadic engagement; (b) engagement is related to the degree of therapeutic alliance; and (c) when the alliance is sufficient, a triadic invariant of engagement emerges. This means that partners regulate and coordinate their behaviors to maintain a stable level of engagement, whatever changes in their conversational organization. Finally, it discusses the potential of this method for describing the interactive aspects of the therapeutic alliance.