It's possible, but very rare. So if you want that, your parens was an Intellego Vim specialist, with an appropriate Focus, and all kinds of handy bonuses. Might just about be possible.
Alternatively, the character had two of them before Opening, and got the third by learning the Ability just after. Or there was a Mystery Initiation involved. There are ways around this, which I think is enough for a rare case.
Note that three Major Supernatural Virtues is generally a very, very bad choice for a magus. You'd get much more use out of a Major Hermetic Virtue and half a dozen Minor ones.
In-game, no control. It just converts. Out of game, players should get to choose.