Calculations of ground-state energies and rotational constants are carried out for a variety of van der Waals dimers and trimers formed from Ne, Ar, Kr and Xe. It is found that the existing pair potentials for Ne-Ar, Ne-Kr and Ne-Xe do not adequately reproduce the measured rotational constants of the van der Waals dimers. Modified pair potentials, with equilibrium distances that differ from the originals by less than 1% but give much better rotational constants, are then proposed. Calculations of rotational constants for Ne-2-Ar, Ne-2-Kr and Ne-2-Xe are carried out using pairwise-additive potentials constructed from both the original and the modified pair potentials. The modified pair potentials give much better agreement with experiment for the trimers as well as the dimers. The effect of an Axilrod-Teller triple-dipole term on the rotational constants is considered, and found to be significant, especially for the A rotational constant. However, the best available Ne-Ne potential is not accurate enough to allow unambiguous information on three-body forces to be extracted.