Many aspects of particle transport in tokamaks are not yet clarified. In particular, the existence and nature of an anomalous pinch remain an unresolved issue. From the theory standpoint, two mechanisms leading to an anomalous pinch have been proposed. One is based on turbulent thermodiffusion and predicts a velocity pinch proportional to the gradient of the temperature logarithm Ts/Ts. The second type is often called "Turbulence Equi-Partition" (TEP) and predicts a velocity proportional to the curvature of the magnetic field. This problem has been investigated with 3D fluid turbulence simulations of Ion Temperature Gradient (ITG) modes and Trapped Electron Modes (TEM), thus extending previous results obtained with 2D simulations. Both analytical calculations and numerical simulations are used to clarify this issue. The results are compared to a subset of the JET database, corresponding to RF heated plasmas in L-mode. Particle transport in H-mode is presented in a companion paper.