Rocking curves of reflection high-energy electron diffraction (RHEED) have been measured from the Si(111)-(3 × 3) R30°-Al, -Ga, and -In surfaces at [21 1] incidence. Dynamical calculations of RHEED intensities and reliability-factor minimization have been carried out to obtain optimal surface structures for the three surfaces. Three high-symmetry sites were examined for every adsorbate atom assuming coverage of 1/3 ML. Among these, the fourfold-coordinated terminal-site (T4) model is in good agreement with the experiment if substrate rumpling is considered. In the RHEED method, coordinates of the surface atoms are obtained with accuracy bettter than about 0.1.