Shape the Shot editorial teamUpdated
Camera blocking and composition
A and B begin at the center while C enters later. The original pair stop their talking actions in the second half and C becomes the speaker. The three cameras cover the original relationship, the arrival and the new arrangement.
Shot-by-shot breakdown

0–3 s 1. Original pair
Establish the relationship before the interruption.

3–7 s 2. New arrival
Shift attention toward the entering character.

7–12 s 3. Three-person wide
Clarify the new spacing and facing directions.
Try it in your own scene
Change C’s entrance time, move the stopping point, then bring the ending closer to A or B. Check whether all three positions remain understandable.
About this example
Do three characters need three close-ups? No. Assign each shot a clear information task instead of distributing close-ups equally.