Geometries and energy separations of various low-lying spin-states of the platinum trimer (Pt-3) have been investigated. The complete active space multiconfiguration self-consistent field (CASMCSCF) method followed by large scale multiconfiguration singles plus doubles configuration interaction (MRSDCI) that included up to 4.26 million configuration spin functions were used to compute several electronic states. A relativistic configuration interaction (RCI) technique was employed to compute the spin-orbit effects in different electronic states. Although an equilateral triangular (1)A(1) state was found to be global minimum in the absence of spin-orbit effects, this state was found to be nearly degenerate with the B-3(1)(A(1)) spin-orbit state when spin-orbit coupling was included. The (1)A(1) and B-3(1)(A(1)) states were found to be heavily mixed by spin-orbit coupling. We also compared our MRSDCI results with the density functional as well as Moller-Plesset second order perturbation calculations. The dissociation and atomization energies have been computed and compared with experiment.