Magic Square of 12...
... or, 'magic square' minus one?
No, a mathematical magic square is of indefinite size, not only 3. So this is specifically a '3' magic square. Minus one, would be a magic square of two...?
Wikipedia's cultural notes seem to point toward Saturn or turtles as another possibility...
Since I can't think of anything Touhou that would really point toward Saturn, the asteroid belt, or Mars (possible Saturn-1s), I'm gonna guess Reimu as a vague 'Genji-1'.