In the redundant signals task, participants respond in the same way to stimuli of different sources, for example, to auditory as to visual signals. The stimuli are presented either alone (single signals) or in combination (redundant signals). In this task, one often observes substantially faster responses to redundant signals than to single signals. Different mechanisms may underlie this so-called redundancy gain, including separate activation and coactivation models. The race model inequality (Miller, 1982) is typically used to distinguish these models. In this article, we consider the generalization of Miller's inequality to redundant stimuli from three sources (e.g., auditory, visual, tactile). We review the approaches from the literature, and derive several inequalities that have been used to investigate interactions between three sensory modalities. We discuss the limitations and generality of a model which assumes up to three distinct signals representing pairwise interactions of sensory systems, and outline statistical approaches for testing the inequalities in empirical observations.