Arnaud de Mesmay, Yo'av Rieck, Eric Sedgwick, Martin Tancer. 3 is NP-hard. In Artur Czumaj, editor, Proceedings of the Twenty-Ninth Annual ACM-SIAM Symposium on Discrete Algorithms, SODA 2018, New Orleans, LA, USA, January 7-10, 2018. pages 1316-1329, SIAM, 2018. [doi]
Abstract is missing.