Using an indirect proof:
Assume that the figure is a trapezoid.
All trapezoids are quadrilaterals.
All quadrilaterals' interior angles add up to 360° because any n-gon's interior angles add up to 180(n-2)°.
We are given that the trapezoid has three right angles.
All right angles are 90°, thus these right angles have a total measure of 270°.
We can conclude fourth angle must be 90°.
If it has four right angles, it is a rectangle.
Rectangles have two sets of parallel sides.
However, trapezoids have exactly one set of parallel sides.
Alas, our figure cannot be a trapezoid.
