Here's my guess at the answer...it is a trick question: the drawing is of only a triangle, fulfilling the criteria of 3 corners, the 'square' is in the maths i.e. Pythagoras theorem
Quote from some maths site:
The Pythagorean theorem states that for any right triangle the sum of the squares of its two legs equals the square of its hypotenuse (a2 + b2 = c2). Or we could frame it as the sum of the square of the horizontal and the square of the perpendicular equals the square of the hypotenuse. We know this formulation was known before Pythagoras - there is evidence in ancient Egyptian work, ancient China (the Chou Pei manuscript), and the megalith builders. Regardless, it is attributed to Pythagoras and two hundred years later Euclid compiled his "Elements of Mathematics" where this particular proposition is found in Book One.
(47th) In every right angle triangle the square on the hypotenuse is equal to the sum of the squares on the other two sides.