Using a constrained molecular modeling method, we have generated DNA (D), RNA (R), and hybrid DNA/RNA triple-helical structures. Starting with the reference frame defined by the X-ray fiber diffraction model of the poly(dT).poly(dA)+poly(dT) triple helix, where "." denotes Watson-Crick pairing and "+" indicates Hoogsteen pairing, we have exhaustively sampled the arrangements of the sugar-phosphate backbone that connect adjacent bases on each of the three strands. We focus attention on regular polymer models constructed from conformationally identical repeating units. Structures free of local steric constraints are combined in all possible ways to build short triplexes, and overall triple helix energies are computed under several different dielectric environments. The predicted ordering of Pyr.Pu+Pyr triplex stabilities. D.D+D > D.D+R > D.R+D > D.R+R > R.D+D > R.D+R > R.R+D > R.R+R, agrees qualitatively with measured stabilities of triplexes containing DNA and/or RNA single strands in gel electrophoresis, NMR, X-ray, and thermodynamic studies. The relevance of the computed data to other triple helical simulations is also discussed.