A crystal plasticity based finite element model incorporating deformation twinning is applied in 3D to simulate the stress field around a twin lamella in a metastable β titanium alloy. Compared with conventional 2D simulations, the stress field determined by the 3D simulation shows better agreement with that measured experimentally using high resolution electron backscattered diffraction. Reasons leading to the different results in 2D and 3D simulations are discussed. It is also discussed in which circumstances 2D simulation is sufficient, and in which 3D is necessary. © Published under licence by IOP Publishing Ltd.