To balance a redox reaction, we do as follows:
We balance the O atoms by adding H2O and adding H+ to balance the H.
NO + 2H2O = NO3- + 4H+
We balance the charge by adding electrons.
NO + 2H2O = NO3- + 4H+ + e-
Fe3+ + e- = Fe2+
Adding the two half reactions,
NO + 2H2O = NO3- + 4H+ + e-
Fe3+ + e- = Fe2+
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NO + Fe3+ + 2H2O = NO3- + Fe2+ + 4H+