High level ab initio calculations are carried out on diacetamide-X (DA-X) dimers, X=HCN, CH3OH. The dimers are used as model systems to investigate the energetics and cooperative phenomomena in intermolecular three-center hydrogen-bond (H-bond) interactions relative to two-center H-bond interactions. The trans-trans conformer of diacetamide is chosen as a suitable model for intermolecular three-center H bonding where one H atom is interacting with two acceptor atoms. The proton-acceptor atoms are rigidly held in the same molecule. For both model systems, it is found that the calculated interaction energy per H bond is appreciably smaller in the three-center than in the two-center H-bond dimers, suggesting possibly a general characteristic of intermolecular three-center H bonds, namely, a negative cooperativity. More importantly, it is found that frequency shifts, intensity factors, bond lengths, and H-1 nuclear magnetic resonance chemical shifts all support the energetic calculations in that the intermolecular three-center H-bond dimers exhibit marked negative cooperative effects. Despite the negative cooperativity, the three-center DA-HCN dimer is actually energetically favorable over the two-center counterpart, whereas the three-center DA-CH3OH dimer is energetically unfavorable over the two-center counterpart.