The optical conductivities of single crystals of ScAl3, LuAl3, and YbAl3 were measured by spectroscopic ellipsometry in the energy range of 1.5–5.5 eV. The conductivity spectra of all show peaks between 1.5 and 2.0 eV originating from interband transitions. Two additional peaks around at 2.9 and 4.2 eV are observed for ScAl3, and a weak shoulder is observed around at 2.8 eV for LuAl3. Band structure, density of states, and optical conductivity were calculated using the full-potential linear augmented plane wave method. The calculated conductivity spectrum for ScAl3 shows peaks around at 2.7 and 4.3 eV. The distinctive feature of the optical conductivity for ScAl3 comes from the extraordinary large Sc d unoccupied density of states compared to those of Yb d and Lu d. Oxidation effects on the optical properties of the sample were studied using a three-phase model. The calculated optical conductivity of the clean surface using the three-phase model is larger than that of the oxidized surface.