A diatomics-in-molecules (DIM) potential energy surface (PES) is constructed for the excited triatomic system Ar-3*. This PES is used for a semiclassical study of the formation and collisional vibrational relaxation of Ar-2* excimers by simulation of the collision process Ar(S-1(0)) + Ar2*((3) Sigma(u)(+)). Evidence is presented for the existence of relatively long-lived Ar-3* complexes which appear to be destroyed in a series of collisions with Ar(S-1(0)); upper and lower boundaries are established for the vibrational relaxation process frequency of Ar-2*.