Cutting insert, cutting tool, and method for manufacturing machined product

A cutting insert, comprising:

• an upper surface having a polygonal shape;
• a lower surface located on a side opposite to the upper surface;
• a lateral surface located between the upper surface and the lower surface;
• and
• an upper cutting edge located on an intersection of the upper surface and the lateral surface, wherein
• an imaginary straight line passing through a center of the upper surface and a center of the lower surface is a central axis,
• the upper surface comprises
• a first corner,
• a second corner, and
• a first side that is connected to the first corner and the second corner and is inclined so as be closer to the lower surface as going from an end part being connected to the first corner toward an end part being connected to the second corner,
• the lateral surface comprises a first lateral surface being connected to the first side,
• the first side comprises
• a first part being connected to the first corner,
• a second part being connected to the second corner, and
• a third part located between the first part and the second part,
• the first lateral surface comprises
• a first region being connected to the first part,
• a second region being connected to the second part, and
• a third region being connected to the third part,
• the first region comprises, in a cross section orthogonal to the first side as viewed from above,
• a first upper region that is connected to the first part and is inclined outward as going away from the first part, and
• a first lower region that is connected to the first upper region and is parallel to the central axis,
• the second region comprises, in a cross section orthogonal to the first side as viewed from above,
• a second upper region that is connected to the second part and is inclined inward as going away from the second part, and
• a second lower region that is located closer to the lower surface than the second upper region and is parallel to the central axis, and
• the third region comprises, in a cross section orthogonal to the first side as viewed from above,
• a third upper region that is connected to the third part and is inclined outward as going away from the third part,
• a third lower region that is located closer to the lower surface than the third upper region and is located more outside than the third upper region and is parallel to the central axis, and
• a concave part that is located between the third upper region and the third lower region and is recessed inward from an intersection of an imaginary extension line of the third upper region and an imaginary extension line of the third lower region.