CP3 despite conventional wisdom has been very good when the team has needed him to step up in the playoffs. Despite losing in 6 games to the Bucks, he averaged 22/3/8 on 55/52/75 in the finals despite playing through an injury.
Devin Booker who was the first option averaged 28/3/4 on 54% TS.
I think Isiah has to be #3 over CP3