  The 3N forces due to pi-rho, pi-sigma and pi-omega exchanges following from
the nucleon Born diagrams and diagrams with an intermediate N^{*}(1440) are
re-analyzed. The cancellation between pi-sigma and pi-omega forces is rather
sensitive to the values of the coupling constants and to the form of the piNN
vertex. Experimental uncertainties in the parameters of the TM pi-pi potential
are assessed. They lead to uncertainties in theoretical predictions of the
triton binding energy of about 0.4 MeV. The low-energy limit of pi-sigma and
pi-omega potentials is performed. It defines the coupling constants of
effective contact piNNNN vertices, which are compared with the corresponding
contact vertices of Chiral Perturbation Theory.
