Three-dimensional sloshing in a bi-lobe tank under arbitrary rotation is simulated using a code developed using the flux-difference splitting scheme for variable density incompressible fluids and the hybrid Cartesian/immersed boundary method. The material interface is regarded as a moving contact discontinuity and is captured using a free surface capturing method derived from the Riemann solver, without any additional treatment along the interface. The boundary condition for the arbitrary motion of the bi-lobe tank, which contains a thin partition between two partially overlapping cylindrical tanks, is handled with ease by using the hybrid Cartesian/immersed boundary method. The computed time evolution of the interface is compared with the snapshots taken during the experiments on sloshing caused by the sway motion of the bi-lobe tank. Good agreement is observed between the computational and experimental results. The validated code is used to simulate three-dimensional sloshing in the bi-lobe tank that is subject to combined pitch and roll motions. A rotational vector is used to locate the Lagrangian points of the unstructured surface grid according to the motion of the tank. Grid independence tests are carried out using three different size grids. Violent three-dimensional sloshing ensues with an increase in the angular velocity of rotation.