sabbe sankhārā aniccā
All formations (conditioned) are impermanent
sabbe sankhārā dukkhā
All formations (conditioned) are suffering
sabbe dhammā anattā
All phenomena (unconditioned—i.e., including Nibbāna—and conditioned things) are not-self
Nibbāna is not included in the first two, for the reasons that you described, but it is a dhamma (it is phenomena) and therefore is classified in the third characteristic (all things are not-self).