The dynamics of x-line formation and evolution in 3D magnetic reconnection is studied using a fully kinetic approach. An x-line of small length is initialized using a perturbation localized in the current direction. The electrons and ions drift diamagnetically along the current direction of the initial x-line and are further accelerated by the reconnection electric field. The electron and ion motion is in opposite directions and each species extends one end of the x-line. Several predictions based on this picture are formulated and studied and confirmed under parameter variation. Expansion can proceed at a significant fraction of the Alfven speed, in both directions.