In a rotary cutting tool used for milling operations, a reversible indexable cutting insert is removably secured in a cutting body. The cutting insert has two opposing end surfaces interconnected by a continuous peripheral surface, including three side surfaces alternating with three corner surfaces. The side and corner surfaces intersect with both end surfaces at side and corner edges, respectively, with each side edge having a major cutting edge, and each corner edge having a corner and minor cutting edge. Each major and minor cutting edge slopes away from first and second end points, respectively, of its mutually associated corner cutting edge, towards a median plane. A first imaginary straight line extending perpendicular to the median plane and intersecting any one of the corner cutting edges at any point along its length except the second end point, passes through the median plane inside an insert boundary line.