What do you refer to an alternation between 3 things? or the like
I think the commonest way to describe movement through a cyclical pattern of more than 2 elements would be "rotation". Your example is a bit non-standard, because there are really only 2 elements, but the way in which they repeat is a triplet. Still, I think the word "rotation" would carry the right meaning. The pattern is a 3-step rotation among the elements 1-1-2.