The off-shell ambiguity of the relative motion function in the resonating group model is discussed. It is shown that off-shell transformations can be used to enhance and reduce three-body force effects in three-cluster systems. The superposition principle of forces is discussed as a criterion for the selection of a physical off-shell behaviour of the relative motion function. As an example the three-cluster system npe - is treated in the framework of a six-quark plus electron resonating group model. It is shown that, at the energy of the deuteron, the three-body force is very small in the version of the resonating group model where the relative motion function is transformed with the square root of the norm operator whereas other types of relative motion functions which are discussed in literature, especially those with a short-distance node, give uncomfortably strong three-body forces.