Following up Benoît Kloeckner's remarks, I include two images below, the first from the paper, "Least-perimeter partitions of the disk into three regions of given areas" (arXiv abs link), the second from D'Arcy Thompson's On Growth and Form:
![3partition][1]
![DarcyThompson][2]
Thompson explains the circumstances in which $120^\circ$ angles are preferred in some regions of the wing over the more prevalent $90^\circ$ angles. The three arcs in the disk in Fig.1 meet at $120^\circ$, and meet the circumcircle at $90^\circ$.