Three-dimensional simulations of collisionless magnetic reconnection in a current sheet configuration are presented. The study consists of two parts: a simulation of a current sheet with an initial perturbation and another model run without it. Particular emphasis is on the questions (1) whether magnetic reconnection remains two-dimensional if it initiates so, and (2) whether reconnection favors a patchy, localized mode over long, extended reconnection regions. Our study suggests that results from 2.5-dimensional models of reconnection are not substantially altered by including variations in the third dimension. In fact, an initially random patchy reconnection structure appear to self-organize into a more coherent quasi-two-dimensional structure. As in 2.5-dimensional reconnection studies, the dominant contribution to the dissipation stems from off-diagonal terms of the electron pressure tensor in our simulations. The dominant term is related to oscillatory electron motions between the regions of newly reconnected magnetic field. ¿ 2001 American Geophysical Union