999

Number
$999$ (nine hundred and ninety-nine) is:


 * $3^3 \times 37$


 * The $8$th Kaprekar number after $1$, $9$, $45$, $55$, $99$, $297$, $703$:
 * $999^2 = 998 \, 001 \to 998 + 001 = 999$


 * The smallest integer which is the sum of a set of $3$ three-digit primes using all $9$ digits from $1$ to $9$ once each is $999$:
 * $149 + 263 + 587 = 999$


 * The smallest multiple of $27$ whose digits add up to $27$:
 * $999 = 37 \times 27$; $27 = 9 + 9 + 9$