We describe the de novo design and characterization of a three stranded antiparallel beta-sheet (peptide 1-24) and investigate the interplay between the two sets of weak interactions that occur at the interfaces between strands 1 and 2, and strands 2 and 3. We show by CD and NMR that peptide 1-24-folds into a three stranded sheet in aqueous methanol, and that the folded conformation of the C-terminal hairpin is more highly populated than the isolated beta-hairpin (peptide 9-24). Both peptides have sigmoidal melting curves but a much broader transition is observed for the hairpin which also has a lower T-m (278K versus 298K); fitting to a two-state model gives the thermodynamic parameters for folding, which in both cases is enthalpy-driven. At 298K the three stranded sheet is approximate to 50% populated, while only approximate to 20% of the hairpin is folded. Despite a relatively small difference in stability between the sheet and hairpin, the former appears to have a much better defined structure in terms of both interstrand main chain and side chain interactions which we equate with a more extensive network of cooperative weak interactions. We have calculated an ensemble of 28 structures of the three-stranded alpha-sheet (backbone RMSD to the mean 1.3 +/- 0.2 Angstrom) using a combined torsion angle-driven distance geometry and molecular dynamics simulated annealing protocol, which reveal a right-handed twisted conformation consistent with beta-sheets found in native proteins.