Mg3(PO4)2 + 2 H2 → 3 Mg + 2 H2PO4
This is an oxidation-reduction (redox) reaction:
2 PV - 2 e- → 2 PVI (oxidation)
4 H0 - 4 e- → 4 HI (oxidation)
3 MgII + 6 e- → 3 Mg0 (reduction)
Mg3(PO4)2 is a reducing agent, H2 is a reducing agent, Mg3(PO4)2 is an oxidizing agent.