A model for describing three-cluster cofigurations of nuclei is formul
ated in terms of the algebraic version of the resonating-group method.
The model is applied to six-nucleon systems represented as an alpha p
article and two nucleons. The results indicate the importance of three
-cluster modes in both bound and continuum states of He-6 and Li-6. In
particular, a neutron halo in He-6 is revealed.