See explanation below.
Work Step by Step
If you write the sequence of molecular orbitals for the H2 molecule, the first two are a bonding sigma one and its antibonding equivalent. If you put in two electrons, they go to the bonding orbital and you obtain the regular H2 molecule. Adding another electron, this would go in the antibonding orbital but, since there would be two bonding electrons and an antibonding one, the H2(-) molecule could still exist. But if you must allocate yet another electron, you end with two electrons in a bonding orbital and two in an antibonding one - these “cancel” each other and the “molecule” has no bond to keep it united.