We have performed three-dimensional MHD simulations of magnetic reconnection to investigate the structure of a reconnection jet. Our simulation results show that the leading edge of a reconnection jet deforms to put on a significantly complicated structure. In a randomly perturbed environment, smaller scale structure develops rapidly and finally the front of the reconnection jet becomes turbulent. In contrast when the initial perturbation is composed of a single sinusoidal mode, coherent magnetic bubbles are generated. The magnetic bubbles elongate significantly when the wavelength is small giving onset to a secondary instability that may subsequently lead to turbulence. We interpret the deformation of the jet front to be due to excitation of the Rayleigh-Taylor/ballooning-like instability and emphasize the significance of the three-dimensionality in considering the dynamics of a reconnection jet.