A repeating decimal with a repeating cycle of three digits can be expressed as a rational number with a denominator of 999.
For example, if you have a repeating decimal like \(0.abcabcabc...\), it can be represented as:
\[ \frac{abc}{999} \]
where \(abc\) is the three-digit number formed by the repeating digits. This is because the decimal repeats every three digits, leading to a factor of \(999\) in the denominator. Therefore, the correct answer is 999.