Three-dimensional (3-D) nonlinear simulations of collisional drift-wave turbulence are presented. Results for the Hasegawa–Wakatani equations (without magnetic shear) in 3-D are compared to former two-dimensional (2-D) simulations. In contrast to the 2-D system the 3-D situation is completely dominated by a nonlinear drive mechanism. The final state of the system is sensitive to the configuration of the computational grid since the sheared flow develops at the longest scales of the system. When magnetic shear is included, the system is linearly stable but the turbulence is self-sustained by basically the same nonlinear mechanism. Magnetic shear limits the size of the dominant eddies, so the system evolves to a stationary turbulent state independent of the computational box. Finally, it is shown that the level of turbulence in the system with magnetic shear depends sensitively on the size of the effective Larmor radius ρs compared with the characteristic transverse scale length of the eddies.