Regardless of what Chinese characters are used, the letters indicating the third official are read 'Jo', which is considered to have come from the Chinese reading of '丞' (Jo); '丞' was originally the title for the third official in some government offices in Tang dynasty.
三等官は文字にかかわらず「じょう」と訓ぜられるが、これは唐の一部官庁で三等官の呼称とされていた「丞」の借音とされる。