Which form of chromium oxide is formed naturally in the atmosphere?
An investigation of the phase diagram reveals that the only stable chrome oxide near room temperature is $\ce{Cr2O3}$.  $\ce{Cr3O4}$ appears, but only near 2000K.  There is no stable chrome analogue to $\ce{FeO}$ on the phase diagram. 

Thus, the one stable oxide that will form under normal atmospheric conditions is $\ce{Cr2O3}$. However, note that it is easy to find mixed metal-oxygen compounds with high mutual solubility. 

If you are interested in the thermodynamics of the $\ce{Cr-O}$ system, you could start with M. Kowalski and P.J. Spencer, Calphad 19(3) 229-243 (1995), which covers the $\ce{Cr-O, Fe-O}$ and $\ce{Ni-O}$ systems