You mean NO3−I assume. It is non polar because it is a symmetrical trigonal planar structure.
I assume you mean NO3−I – (the nitrate anion); there is no NO3−I molecule.
It is non-polar because it has a trigonal planar structure and the symmetry means that there is an even distribution of electron charge density over the three N−O bond. Whilst often draw the structure of NO3−I as a Lewis structure comparison double and single bonds, the reality is that the three N−O bonds are actually identical. The delocalised electron system results from a combination formula charge with each oxygen carrying −2/3 of a charge, and the nitrogen carrying a+1 charge. Adding these up (3×−2/3)+1=−1 which is the formal charge of the nitrate ion.