We have investigated the defect structures and potential of mean force (PMF) of three colloidal spheres immersed in a nematic liquid crystal (for linear and equilateral-triangular configurations), using a coarse-grained theory for the tensor order parameter. At large separations, each sphere is surrounded by an equatorial Saturn ring defect; at short separations, the theory predicts a drastic reorganization of the disclination lines: additional disclination rings are observed in planes perpendicular to the original ones. For both types of configurations, the PMF is attractive and always greater than a pairwise sum of binary PMFs. Comparing the PMFs of linear and equilateral-triangular configurations, we have found the triangular configuration to be more stable than the linear configuration. (c) 2005 Wiley Periodicals, Inc.