The x coordinate of the equilateral triangle will be located exactly in the middle of the other two coordinates (applicable only when one side is along the x axis). Thus, the x coordinate is 1.5.
The y coordinate of the triangle will be √3/2 times the length of the side (from the vertices given, i.e. (0,0) and (3,0), the length of one side is 3 units).
Thus the y coordinate is √3/2 * 3 = 3√3/2.
But the equilateral triangle can have its tip pointed up (like a mountain ^) or its tip pointing down (like a conical well v). So the y coordinate can also be -3√3/2.
Thus, the coordinates of the third point are either (1.5 , 3√3/2) or (1.5 , -3√3/2).
If you have any doubts, please comment below.