Has the molecular diatomic oxygen trianion been observed or predicted?
According to "Small Multiply Charged Anions as Building Blocks in
Chemistry" Acc. Chem. Res. 1996, 29, 497-502


  it is concluded that all isolated diatomic dianions or
  other multiply charged diatomic anions are not stable
  to loss of an extra electron or fragmentation in their
  ground electronic states


So even $\ce{O2^{2-}}$, let alone $\ce{O2^{3-}}$ cannot exist in isolation.