what is the oxidation no of I3-

Answer 1

I₃^- is
I₂ + I^-.
The oxidation number of the ion, as shown, is -1 which makes each I a -1/3. I often balance the equation as I2, then add the I^- as needed. It saves wear and tear on the fractions.

Answer 2

This would be correct for i3-

Answer 3

-1/3