Controlling a triangular flexible formation of autonomous agents. * *The work of Hector Garcia de Marina was supported by Mis-trale project, http://mistrale.eu. The work of Zhiyong Sun and Brian Anderson was supported by the Australian Research Council Grants DP160104500, DP130103610 and the Prime Ministers Australia Asia Incoming Endeavour Postgraduate Award. The work of Cao was supported in part by the European Research Council (ERC-StG-307207) and the Netherlands Organization for Scientific Research (NWO-vidi-14134).