Thanks. I missed the Cuban part of the discussion, only focused on the American mother / born in Canada part. I never considered the triple citizen at birth, only dual citizen.
-PJ
No sweat. It's natrual to view events with the benefit of hindsight, and to jump to the conclusion that Ted Cruz was mostly US citizen at birth. But that conclusion is driven by how he was raised, not by his circumstance of birth. If his parents had moved to Cuba, he'd be "mostly Cuban." The child follows the parents, until he reaches the age of majority, at which time he can expatriate himself by action or by formal renunciation (depends on the country being expatriated from, see the Kawakita case for an example of dangers of dual citizenship).
It may be that Ted's claim to Cuban citizenship was extinguished for him, by his parents, if and when his parents chose to claim his US citizenship by having it adjudicated by a Consular office or the State Department, once stateside.