Which types of quadrilateral have exactly three right angles?

have ##4## sides and ##4## angles. The exterior angles of any convex polygon (ie no interior angle is less than ##180## degrees) add up to ##360## degrees (##4## right angles). If an interior angle is a right angle then the corresponding exterior angle must also be a right angle (interior + exterior = a straight line = ##2## right angles).

Here ##3## internal angles are each right angles, so the corresponding ##3## external angles are also right angles, making a total of ##3## right angles. The remaining external angle must be ##1## right angle ##(=4 - 3)##, so the remaining ##4th## interior angle is also a right angle.

Therefore, if ##3## internal angles are right angles, the 4th angle must also be a right angle.

So no quadrilaterals have exactly ##3## right angles.