The first number represents each lens diameter. This is the distance between the vertical sides of a rectangle containing the lens shape.
The second number is the width of the bridge. This is the minimum horizontal distance between the nasal surfaces of the rims on a plastic or metal frame, or the minimum horizontal distance between the nasal surfaces of the lenses in rimless frames.
The third number is the arm length. This is measured from the dowel point to the extreme end of the side, this is made up of the length to the bend and the length of the drop to give the overall length of side.