  I consider a Hubbard-Anderson model which describes localized orbitals in
three different atoms hybridized both among themselves and with a continuum of
extended states (ES). Using a generalized Schrieffer-Wolf transformation, I
derive an effective Kondo model for the interaction between the doublet ground
state of the isolated trimer and the ES. For an isoceles trimer with distances
a, l, l between the atoms, the Kondo temperature is negligigle for l < a and
has a maximum for finite l > a when a is small. The results agree with
experiments for a Cr trimer on Au(111). [T. Jamneala et al., Phys. Rev. Lett.
87, 256804 (2001)].
