All of C-74-based endohedral metallofullerenes (EMEs) are found to be monometallofullerenes with the same D-3h(14246)-C-74 cage so far. An opening question is whether other C-74 cages could survive during the production of some novel C-74-EMFs. Theoretically, we studied the trimetallic endohedral fullerene Er3C74, the existence of which had been proven without any further characterizations. Two thermodynamically stable Er3C74 isomers were obtained, both of which could be expressed as Er-3@C-74, meaning that previously synthesized Er3C74 is indeed an endohedral trierbium fullerene. Besides the isomer with well-known D-3h(14246)-C-74 cage which obeys isolated pentagon rule (IPR), another one possesses the C-1(13771)-C-74 cage with two adjacent pentagons. Notably, it is the first time an endohedral metallofullerene containing the C1(13771)-C-74 cage has been reported. Frontier orbitals analysis, bonding analysis in terms of quantum theory of atoms-in-molecule (QTAIM) and Mayer bond order, together with two-dimensional maps of electron localization function (ELF) and Laplacian of electron density of Er-3@D-3h(14246)-C-74 and Er-3@C-1(13771)-C-74 show obvious covalent interactions not only between metallic atoms and carbon cage but also among three erbium atoms. Finally, simulated IR spectra of Er-3@D-3h(14246)-C-74 and Er-3@C-1(13771)-C-74 were simulated, which should be useful to distinguish those two isomers.