The three-digit number missing from the equation is 998. Strangely, the number jumps from 997 to 999. The number 998,001 is actually part of a small family of numbers with interesting mathematical quirks: the numbers that are the squares of repeating 9s. 998,001 happens to be the square of 999… More here.
Call me a nerd if you like, but I find this oddly fascinating.