Like when iron reacts with water there forms iron oxide and hydrogen but how'll we come to know which of three equation will be right equation for reaction?
$$\ce{Fe + H2O → FeO + H2}$$
$$\ce{2Fe + 3H2O → Fe2O3 + 3H2}$$
$$\ce{3Fe + 4H2O → Fe3O4 + 4H2}$$
The stability of the various iron oxides depends on the oxygen fugacity (or oxygen partial pressure, which is a good approximation to fugacity in near-atmospheric pressures).
$\ce{Fe^3+}$ is the stable form of iron in atmospheric oxygen partial pressures, this in such conditions $\ce{Fe2O3}$ (hematite) will always be the stable product. The lower oxidation state oxides (such as magnetite $\ce{Fe3O4}$ and wüstite ~$\ce{FeO}$) can only stably form in deep earth environments, or in special controlled atmosphere reaction chambers.
That said, in normal conditions it is even more likely for iron hydroxides to form hydrated compounds, especially when water is a reactant, or is present. In these cases, you get a zoo of various Fe-oxides-hydroxides, some of which are amorphous (not crystalline), and others are metastable. These compounds are generally referred to as "rust".
