A surface integral equation (SIE) solver is proposed to simulate arbitrarily-shaped threedimensional monoanisotropic metasurfaces. The solver models the metasurface as an infinitesimally thin sheet where generalized sheet transition conditions (GSTCs) are applied to "connect" the fields on two sides of the sheet. Electric and magnetic field integral equations, which are formulated in terms of unknown equivalent electric and magnetic currents on either side of the sheet, are combined with GSTCs. This coupled system of equations is discretized using the Rao-Wilton-Glisson (RWG) functions. The resulting matrix system is then solved for the coefficients of the RWG expansion using an iterative solver. A numerical scheme is developed to compute the electric and magnetic susceptibility tensors in GSTCs from the desired metasurface response. Numerical results are presented to demonstrate the accuracy and applicability of the proposed SIE-GSTC solver.