3 CrIIO + 2 N-IIIH3 3 Cr0 + N20 + 3 H2O

This is an oxidation-reduction (redox) reaction:

2 N-III - 6 e- 2 N0 (oxidation)

3 CrII + 6 e- 3 Cr0 (reduction)

Change the oxidation states