A single-sided three-way indexable milling insert for 90° shoulder milling operations, includes a positive basic shape, a rake surface, a peripheral surface including side abutment surfaces, a base bearing surface and a screw hole connecting the rake and base bearing surfaces. The insert has three main cutting edges which lie along an imaginary equilateral triangle. A material volume VF of the cutting insert and a void volume VS of the insert's screw hole fulfill the condition VS/VF≥0.30.