Homonuclear triatomic molecules contain three of the same kind of atom. That molecule will be an allotrope of that element.
Ozone, O is an example of a triatomic molecule with all atoms the same. Triatomic hydrogen, H, is unstable and breaks up spontaneously. H, the trihydrogen cation is stable by itself and is symmetric. He, the helium trimer is only weakly bound by van der Waals force and is in an Efimov state. Trisulfur (S) is analogous to ozone.