Why not using the simple fact that cos(60°) = 1/2?
Put your dough in the form of a circle, make sure you know the centre point. From there you draw a horizontal line to the right, and in the middle of that line, you draw a perpendicular line up and down. Like this, you find two points, one above and one beneath. Together with the outmost left point of your circle, this forms the edges of the third parts you are looking for (I'm trying to find out how to attach a picture to this answer)
