its impossible to draw a trapezoid with just three right angles.
a trapezoid has 4 sides, which means all the angles inside the trapezoid must add up to 360 degrees.
if you have just 3 right angles (90x3), you already use up 270 degrees. Leaving you with just 90 degrees left, which is also a right angle. That means, there has to be four, if you have at least 3.