A cutting insert of an embodiment of the present invention includes: a polygonal shaped upper surface; a lower surface being identical in shape to the upper surface; a side surface connected to each of the upper and lower surfaces; and an upper cutting edge located at the intersection of the upper surface and the side surface. The upper surface alternately includes three major corners, each having a first interior angle, and three minor corners, each having a second interior angle larger than the first interior angle. The upper cutting edge includes: a corner cutting edge; a minor cutting edge inclined toward the lower surface as separating from the corner cutting edge at a first inclination angle on the basis of a vertical plane perpendicular to a central axis extending between the upper and lower surfaces; and a major cutting edge inclined toward the lower surface as separating from the minor cutting edge at a second inclination angle larger than the first inclination angle on the basis of the vertical plane. The corner cutting edge, the minor cutting edge and the major cutting edge are located sequentially from a first major corner of the three major corners to each of first and second minor corners of the three minor corners, both of which are adjacent to the first major corner. A cutting tool with the cutting insert, and a method of manufacturing a machined product by using the cutting tool are also provided.