For the design of the Dutch free electron laser FELIX [P.W. van Amersfoort et al. this issue] computer simulations are done with two different codes, G3DH and TDA, to estimate three dimensional effects in the amplification process in the undulator. G3DH [E. Jerby and A. Gover, Nucl. Instr. and Meth. A272 (1988) 380] solves a 3D matrix gain dispersion equation, TDA [T.-M. Tran and J.S. Wurtele, to be published] is a nonlinear 3D particle simulation code. Both codes take into account energy spread and emittance of the electron beam, Rayleigh length and waist position of the e.m. beam, and the planar undulator form. The finite length of the electron beam microbunches is not considered. The two methods to calculate the evolution of the beam profiles along the undulator are compared. Beam guiding and other effects are presented for various input conditions.