  We present results from numerical simulations of three different 3d
four-fermion models that exhibit Z_2, U(1), and SU(2) x SU(2) chiral
symmetries, respectively. We performed the simulations by using the hybrid
Monte Carlo algorithm. We employed finite size scaling methods on lattices
ranging from 8^3 to 40^3 to study the properties of the second order chiral
phase transition in each model. The corresponding critical coupling defines an
ultraviolet fixed point of the renormalization group. In our high precision
simulations, we detected next-to-leading order corrections for various critical
exponents and we found them to be in good agreement with existing analytical
large-N_f calculations.
