The geometric and electronic structures of phosphaethyne trimers were studied using density functional theory (B3LYP/cc-pVTZ) as well as CBS-QB3, CCSD(T), and complete active space self-consistent field (CASSCF) computations. The effect of the substituents was also investigated. Our aim was to identify the low-energy equilibrium structures with synthetic interest; therefore, we were determining minima without any restriction on the chemical structure. The global minimum of the C3P3H3 potential energy surface is confirmed to be the 1,2,3-triphosphinine. The next energetically stable isomer is a potentially synthesizable target with a P-3 ring, which is only 34 kJ/mol above the global minimum. Formation of phosphaethyne trimers by reaction of phosphaethyne and 1,4-diphosphatriafulvene has been determined. This suggests that the thermodynamically controlled product is the 1,3,5-triphosphinine, while 1,3,6-triphosphafulvene is the kinetically favored product. For the latter a biradical reaction pathway also exists. Analysis of the effect of the substituents shows that the reaction barriers increase with the size of the substituent. Using the bulky 2,4,6-tri-tert-butylphenyl (supermesityl) substituent, the relative stabilities of 1,3,5-triphosphinine and 1,3,6-triphosphafulvene are reversed, and the latter is expected to be the most stable phosphaalkyne trimer with bulky substituents. The head-to-tail reaction of phosphaethyne and phosphanylidene carbenoid yields 1,3,6-triphosphafulvene with very small barriers, while 1,4-diphosphafulvene is expected to be a byproduct. The results are in good agreement with the experimental findings.