  We discuss effective field theory treatments of the problem of three
particles interacting via short-range forces. One case of such a system is
neutron-deuteron scattering at low energies. We demonstrate that in attractive
channels the renormalization-group evolution of the 1+2 scattering amplitude
may be complicated by the presence of eigenvalues greater than unity in the
kernel. We also show that these eigenvalues can be removed from the kernel by
one subtraction, resulting in an equation which is renormalization-group
invariant. A unique solution for 1+2 scattering phase shifts is then obtained.
We give an explicit demonstration of our procedure for both the case of three
spinless bosons and the case of the doublet channel in nd scattering. After the
contribution of the two-body effective range is included in the effective field
theory, it gives a good description of the nd doublet phase shifts below
deuteron breakup threshold.
