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III is the Roman numeral for the number three, formed by three I characters. It is used in many contexts to mark order or sequence, such as chapters, volumes, or versions, as well as regnal numbers for monarchs and other figures, and in date-style inscriptions when Roman numerals appear.

In music theory, III denotes the triad built on the third degree of a diatonic scale. Roman numerals indicate chord quality: uppercase numerals typically signal major triads. In major keys the III triad is usually major, while in minor-key contexts the quality can vary depending on the scale form used.

In science and medicine, III is used to indicate oxidation state +3, as in Fe(III) for ferric iron. It also appears in regulatory classifications, such as Class III medical devices, which are regarded as high risk and subject to stringent premarket approval and oversight.

In culture and media, III commonly marks the third installment or volume of a work, including films, books, and video game series. The designation helps audiences distinguish between successive entries with ongoing narrative or thematic continuity.

Overall, III functions as a versatile symbol across disciplines, signaling order, a specific musical harmony, chemical state, regulatory category, or sequential part of a larger work.