Without calculations, since the three vectors have magnitude 3.00, you can deduce that the three angles will be 60 degrees each.
However, you also know that since B points downwards, it goes beyond the x-axis. Since the angle at the origin is 60 degrees and the angle that A makes with the positive direction of the x-axis is 30 degrees, you can deduce that the angle that the resultant makes with the positive direction of the x-axis is (60-30) degrees = 30 degrees.
You get me here?
Now, if you move the resultant to the origin, you will notice that the origin makes an angle of (180-30) degrees = 150 degrees with the +ve direction of the x-axis.
Since the question asks you to report all angles counterclockwise from the positive x axis, you simply start from the x-axis, and move counterclockwise until you reach your resultant.
The answer is 150 degrees.