Suppose you mean the three interior angles of a triangle.
The sum of those 3 angles is 180 degrees.
Since one angle is 90 degrees and the second angle is 80 degrees, the third angle will be 180 - 90 - 80 = 10 degrees.
Hence, the third angle will be 10 degrees if we are talking about a triangle here. If this is not a triangle, you may want to restate the full question.