分之 is a construction — not a word — that goes between two numbers to form a fraction. The pattern is numerator + 分之 + denominator in Chinese, which is the reverse of English order: 三之二 means 'two-thirds' (literally 'out-of three parts, two'). You must read the second number as the denominator, then the first number as the numerator. This works for any integers and for decimal readings too (百分之三 = 'three percent').