This study presents a numerical method to describe and predict the phenomena of three-dimensional nonlinear liquid sloshing problem of a tuned liquid damper (or TLD) to any kind of forced motion. The three-dimensional boundary element method with the second-order Taylor series expansion and Lagrangian description is established and used to compute the position and other quantities of the liquid particles in the liquid domain and on the free surface. The calculations of transient solution (or the time history) of the free surface movement and the base shear force from hydrodynamic pressures of a three-dimensional rectangular or cylindrical TLD tank subjected to horizontal harmonic forced oscillation, as two examples, are included in this paper for demonstration and discussion.