III is the Roman numeral for 3, so that would mean it's "Iron 3", which means the iron ion has a charge of 3+.
It stands for the charge on the iron ion. In Iron III oxide iron has a 3+ charge.
gallium (III) carbonate - Ga2(CO3)3 gallium (III) oxide - Ga2O3
Yes, boil Chromium (III) oxide with Sulphuric acid and get the dark green Chromium (III) sulphate solid on evaporation. Cr2O3 + 3H2SO4 ---> Cr2(SO4)3 + 3H2O