Narrow bandgap Bi₂Se₃, Bi₂Te₃, and Sb₂Te₃, commonly referred to as classic 3D topological insulators, were studied at room temperature by spectroscopic ellipsometry and optical reflection spectroscopy over the mid-IR-near-infrared photon energy range. Complementarily, Hall measurements were performed. Plasmons in optical loss function and reflection coefficient were identified. The conventional approach based on the high frequency dielectric constant was shown to work well in the description of plasmons in Bi₂Se₃ and Sb₂Te₃ and to fail in the case of a similar compound, Bi₂Te₃. The obtained results are discussed in terms of single- and multivalley approaches to the studied samples with taking the details of the calculated band structure into account.