How can iron form 3 bonds if it has 2 valence electrons?

I'm new to chemistry and understand covalent and ionic bonding a little, but whilst Fe^2+ makes sense to me, Fe^3+ doesn't. Why would it want to donate 3 electrons if the next shell down from the outermost has 14 electrons in it? (I'm thinking of the reaction between iron(III)oxide plus HCl)