How many right angles can a quadrilateral have?

a quadrilateral can have from 0 to 4 right angles.

But it can't have exactly three right angles. The interior angles of a quadrilateral sum to 360 degrees. If it had three right angles and x were the measure of the fourth angle:

3*90+x=360

x=360-270=90

So if it has three right angles, the fourth angle would be a right angle as well.