The first chapter of Fleming's "Molecular Orbitals and Organic Interactions," describes the formation of triatomic hydrogen. In summary, the sigma MO of diatomic hydrogen interacts in phase with the third hydrogen's AO. In consequence, a new bonding and antibonding MO are formed. The antibonding MO of diatomic hydrogen does not interact due to antisymmetry.
Since two electrons will fill the bonding MO and one will fill the antibonding MO, there is a overall bonding interaction. This raises my question as to why triatomic hydrogen is relatively short-lived, a duration of one millionth of a second prior to decomposition.
