Predicting the response of a tidal turbine to unsteady inflow conditions is a challenge for turbine designers, with consequences for fatigue life and manufacturing costs. The unsteady load models currently used are all based on 2D strip-theory methods. However, the assumption of locally 2D flow is not likely to apply to tidal turbines, and the effect of 3D geometry on the unsteady response is not widely known. This study uses a combination of time-stepping and frequency-domain vortex lattice models, together with URANS simulations, to show the effects of 3D geometry on the unsteady load response of a tidal turbine. The effects of steady-state wake rollup and of unsteady distortion of the wake by the inflow are quantified in terms of their impact on both the mean and the unsteady load. Comparisons to predictions from classical 2D aerofoil theory show significant differences between 2D and 3D modelling, especially for torque predictions. The frequency-domain inviscid vortex lattice model agrees well with the URANS result, suggesting that it is an appropriate tool for unsteady turbine hydrodynamic analysis.