What does a trapezoid with three right angles look like?

A trapezoid can't have three right angles. A quadrilateral with three right angles must have a total of four right angles, since a quadrilateral's interior angles add up to 360. 360 - (3*90) = 90, so the fourth angle would have to be right as well. A quadrilateral with four right angles is not a trapezoid; instead it is a rectangle or a square.