Is NO3 Polar or Nonpolar?

Answer: NO3- is based on its geometric structure technically a nonpolar molecule because the different oxygen atoms are dispersed in a symmetrical manner around the central nitrogen atom. However the presence of extra lone pair electrons on two of the oxygens induces regions of partial negative charge on those oxygen atoms. 

Although nitrogen trioxide (NO3) exists, it is so unstable that it has never been isolated in pure form and theoretically only serves as an intermediate in certain reactions. Therefore we will continue this discussion by focusing on the nitrate (NO3-) ion. Within this structure the nitrogen atom has a +1 charge whereas the three oxygen atoms have partial -2/3 charge. The diagram shown below exhibits one possible resonance structure: the second bond will continuously switch b/t the oxygen atoms and end up spending approximately one-third of its time with each. In this manner a partial bond is formed with each oxygen atom. Nitrates commonly form a wide variety of inorganic salts with an ionic character containing many different metal elements. Examples include KNO3 and Fe(NO3)3.

NO3 Ball and Stick Diagram
NO3 Ball and Stick Diagram. Created with Avagadro.
Where is NO3- commonly found in the real world?

There are a wide range of nitrate sources. It is found in our diets via green foods and cured meats. The salt is present within the earth in relatively high concentrations again in salt form. Indeed some bacteria have even evolved to utilize NO3- as a food source. Within industrial uses nitrates feature prominently as fertilizers in agriculture. They are deemed not toxic but there is a small risk of nitrates degrading to nitrites (NO2-) which are a possible carcinogen.