Actually it’s possible. It’s true of both iron and magic keys that they are guaranteed a rare or ultra rare (respectively) or better. I have seen it myself on numerous occasions, that an iron key gives me two commons and an ultra rare, for example, or a magic key gives me two commons and an epic. I’ve also gotten other combinations, such as an iron key yielding three rares, or a common, a rare and an ultra rare; or a magic key yielding two rares and an ultra rare. And usually, when I get something good like an epic or a legendary, I don’t get much else other than commons along with it.
So rejoice! Though the statistical likelihood of getting three such illustrious and sought-after legendaries as the above named is infinitesimally small, it’s still possible!