The effective properties and stress spectrum of a unidirectional three-phase composite under transverse loading are related to the modulus and thickness of the interphase. Stress spectra represent the point stresses in a material as a frequency distribution and thus give information on stress moments and extrema. The stresses in the matrix of a two- and three-phase composite are normally distributed. Increasing the modulus of the interphase leads to a decrease of the mean stress and increase of the breadth of this stress distribution. Increasing the interphase thickness mainly leads to an increase of the breadth of the stress distribution. The results of the present analysis provide a guide to optimizing interphase properties in order to achieve improved off-axis mechanical properties of three-phase composites.