One of the Chinese anti-fraud numerals 大写数字 used on checks, invoices, and financial documents. 叁 replaces 三 because the simple character 三 can easily be altered (e.g., adding strokes to forge a different number). Full set: 壹贰叁肆伍陆柒捌玖拾.
The 厶 at the top is the indexing Kangxi radical for 叁 (formal accounting variant of 三 'three'). It functions purely as a lookup anchor here, not as a semantic contributor - the elaborate formal numeral was built to make forgery-proof financial documents.
The 大 in the middle was added to the simple 三 to bulk out the graph and prevent forgery - an extra stroke to a casual 三 could turn it into 四 or 五, so the formal variant inserts 大 as a tamper-proof body.
The 三 at the bottom carries the actual numeric meaning. 叁 is the 大写 (formal accounting form) of 三, used on cheques and contracts where the simple three-stroke 三 could be tampered with by adding strokes to become other digits.
