B. You write down the same number.
C. Have them write down another 3 digit number. (Don't let them write 999.)
D. You write down the number whose digits are 9-complements to theirs. i.e. If their first digit is 7 your first digit is 2. Try not to let them notice this.
Astound them by quickly computing (A*B)+(C*D).
What's your secret?