J. J. Thomson observed  while experimenting with positive rays. He believed that it was an ionised form of H from about 1911. He believed that H was a stable molecule and wrote and lectured about it. He stated that the easiest way to make it was to target potassium hydroxide with cathode rays. In 1913 Johannes Stark proposed that three hydrogen nuclei and electrons could form a stable ring shape. In 1919 Niels Bohr proposed a structure with three nuclei in a straight line, with three electrons orbiting in a circle around the central nucleus. He believed that  would be unstable, but that reacting  with H could yield neutral H.  Stanley Allen's structure was in the shape of a hexagon with alternating electrons and nuclei.
In 1916 Arthur Dempster showed that H gas was unstable, but at the same time also confirmed that the cation existed. In 1917 Gerald Wendt and William Duane discovered that hydrogen gas subjected to alpha particles shrank in volume and thought that diatomic hydrogen was converted to triatomic. After this researchers thought that active hydrogen could be the triatomic form. Joseph Lévine went so far as to postulate that low pressure systems on the Earth happened due to triatomic hydrogen high in the atmosphere.
In 1920 Wendt and Landauer named the substance "Hyzone" in analogy to ozone and its extra reactivity over normal hydrogen. Earlier Gottfried Wilhelm Osann believed he had discovered a form of hydrogen analogous to ozone which he called "Ozonwasserstoff". It was made by electrolysis of dilute sulfuric acid. In those days no one knew that ozone was triatomic so he did not announce triatomic hydrogen. This was later shown to be a mixture with sulfur dioxide, and not a new form of hydrogen.
In the 1930s active hydrogen was found to be hydrogen with hydrogen sulfide contamination, and scientists stopped believing in triatomic hydrogen. Quantum mechanical calculations showed that neutral H was unstable but that ionised  could exist.  When the concept of isotopes  came along, people such as Bohr then thought there may be an eka-hydrogen with atomic weight 3.  This idea was later proven with the existence of tritium, but that was not the explanation of why molecular weight 3 was observed in mass spectrometers. J. J. Thomson later believed that the molecular weight 3 molecule he observed was Hydrogen deuteride. In the Orion nebula lines were observed that were attributed to nebulium which could have been the new element eka-hydrogen, especially when its atomic weight was calculated as near 3.  Later this was shown to be ionised nitrogen and oxygen.
Gerhard Herzberg was the first to actually observe the spectrum of neutral H, and this triatomic molecule was the first to have a Rydberg spectrum measured where its own ground state was unstable.