Two-dimensional (2D) strip-theory modelling of unsteady gust-aerofoil interaction is standard practice in many industrial applications, but the limits of applicability of 2D unsteady flow modelling on 3D wing and rotor geometries are not well understood. This paper investigates the effects of 3D geometry features, such as finite span, taper, sweep and rotation, on the unsteady lift response to gusts, and the flow-physical differences between 2D and 3D geometries in unsteady flow. A frequency-domain inviscid vortex lattice model is validated and used for the 3D analysis. The results are compared to unsteady transfer functions from 2D linear analytic theory (e.g. Theodorsen and Sears functions). The study agrees with previous research findings that 3D effects are most significant at low reduced frequencies and low aspect ratios, as well as near the wing tips. The driving cause of 3D response is shown to be the wake vorticity: both streamwise and spanwise components of unsteady wake vorticity must be modelled. The study concludes by investigating whether unsteady response of more complex 3D wing and rotor geometries can be represented by the response of a rectangular wing. The results indicate that this is possible for tapered wings and rotating blades, but not for swept wings.