The correct IUPAC name of $\mathrm{K}{}_3[\mathrm{Fe}(\mathrm{CN}){}_6]$ is

The anion is $[\mathrm{Fe}(\mathrm{CN}){}_6]{}^{3-}$, so iron is $x+6(-1)=-3$, giving $x=+3$. An anionic complex takes the '-ate' ending on the metal (ferrate) and the cation is named first without multiplying prefixes, so the name is potassium hexacyanidoferrate(III).