Draw a diagram to support your answer.
90 degrees if the third point lands where I think it does (although you don’t specify) - see attached. I know because they’re be 4 of them if I mirrored a point across to the other side of the circle. If the point did not bisect the other two, then it would take a little more trig, but I could figure it out
Since the endpoints lie opposite to each other and are a part of the diameter, the two arms make a straight line because a diameter is always a straight line. So the angle formed is a straight angle and so it is equal to 180°.
And please do let me know if you have any other doubts!