A triangular cutting insert has first and second surfaces, a midplane, a peripheral surface, with an insert center axis. The peripheral surface has alternating relief flanks and corners. A major cutting edge formed between each relief flank and the first and second surfaces, connects at a corner to a wiper edge included in a bounding plane of the insert. The wiper edge further connects to a transitional slope directed toward the midplane and an adjacent major cutting edge. Viewed along the insert center axis, an insert footprint is symmetrical to each corner bisector, and each major cutting edge is perpendicular to the associated secondary slope. Viewed perpendicular to the bisector, the relief flank is in a peripheral plane, and the transitional slope is between the peripheral plane and the insert center axis. Viewed along the corner bisector, each visible major cutting edge forms a first angle with the bounding plane.