Triatomic hydrogen or H3 is an unstable triatomic molecule containing only hydrogen.
Being unstable, the molecule breaks up in under a millionth of a second.
The molecule can only exist in an excited state.
Vibrations can occur in the molecule in two ways, firstly the molecule can expand and contract retaining the equilateral triangle shape (breathing), or one atom can move relative to the others distorting the triangle (bending).
https://en.wikipedia.org/wiki/Triatomic_hydrogen