Can a parallelogram have 3 right angles?

Depends on your question.

A rectangle is a parallelogram and has 4 right angles.

BUT

A parallelogram can have only a total of one, two, or four right angles.

Mathematically, 3 right angles is 270 degrees.

A parallelogram has 360 degrees.

360 - 270 = 90 degrees = right angle.