The spectra of sixteen optical functions of the group of compounds Mg2X (X – Si, Ge, Sn) in the range of 1—11 eV at 77 K have been determined and compared. Their main features and general regularities have been established and compared. The calculations are performed on the basis of the experimental reflection spectra R(E) using the Kramers–Kronig relations and analytical formulas connecting optical functions. The integral spectra of the imaginary part of the dielectric function e2(E) are decomposed into elementary transition bands in the 1.5–6.0 eV region using an improved nonparametric method of combined Argand diagrams, taking into account the effective number of valence electrons forming individual elementary bands. For Mg2X compounds (X – Si, Ge, Sn), instead of five to six maxima and shoulders in the integral spectra, an average of six times more elementary components of transitions caused by excitonic and interband transitions were revealed. The energies of the maxima and areas of the selected elementary components of the transition bands of three compounds are determined and compared. On the basis of the known theoretical calculations, the supposed nature and localization for the revealed elementary components of the transitions are proposed. A diagram is proposed for the dependence of the energies of the maxima of the selected transition bands on the lattice parameter of the three compounds.