One of the key processes governing the structure and evolution of astrophysical jets is their interaction with the surrounding medium. A jet can deposit momentum and energy in the ambient medium, and entrain external material. The main physical process responsible for mixing between a jet flow and the ambient medium is the Kelvin-Helmholtz (KH) instability. We have previously analysed the 2D evolution of the axisymmetric modes of a cylindrical jet (Bodo et al 1994) and of the antisymmetric modes of a planar slab jet (Bodo et al 1995). These last are thought to give indications of the 3D evolution of the helical modes of a cylinder, since the linear behavior is very similar. In this contribution we present some preliminary results of fully 3D simulations comparing them with the mentioned 2D results.