Density-functional theory has been used to determine the ground-state geometries and electronic states for homonuclear transition-metal trimers constrained to equilateral triangle geometries. This represents the first application of consistent theoretical methods to all of the ten 3d block transition-metal trimers, from scandium to zinc. A search of the potential surfaces yields the following electronic ground states and bond lengths: Sc3(A1′2,2.83Å), Ti3(E′7,2.32Å), V3(E″2,2.06Å), Cr3(E′17,2.92Å), Mn3(A2′16,2.73Å), Fe3(E″11,2.24Å), Co3(E″6,2.18Å), Ni3(A2″3,2.23Å), Cu3(E′2,2.37Å), and Zn3(A1′1,2.93Å). Vibrational frequencies, several low-lying electronic states, and trends in bond lengths and atomization energies are discussed. The predicted dissociation energies ΔE(M3→M2+M) are 49.4kcalmol−1(Sc3), 64.3kcalmol−1(Ti3), 60.7kcalmol−1(V3), 11.5kcalmol−1(Cr3), 32.4kcalmol−1(Mn3), 61.5kcalmol−1(Fe3), 78.0kcalmol−1(Co3), 86.1kcalmol−1(Ni3), 26.8kcalmol−1(Cu3), and 4.5kcalmol−1(Zn3).