rwndk1 You are right Tzav is rare however it is not one of the 3 rarest (it is fourth).
It is read in a leap year when Pesach is shabbos or Sunday occurs about 13.9%. (In Yerushalyim it might be rarer)
Pinchas is less common (interestingly one year heh-ches-aleph has both at 3.9% this is one of the rarest calendar possibilities.