Formation of trifluoride anion

I wanted to understand mechanism of the formation of $\ce{F3-}$ anion through the molecular orbital theory.

UPDATE - What I really want to know is that when $\ce{F3-}$ is formed from $\ce{F2-}$ and $\ce{F}$, the unpaired electron of $\ce{F}$ enters the anti-bonding orbital $\sigma^*_{\mathrm{2s}}$ then shouldn't the $\ce{F2-}$ ion break up, as all the bonding and anti-bonding orbitals would be filled up, resulting in a bond order of 0?