How many three-digit numbers are there?

Available digits for filling any place = {1, 2, 3, 4, 5, 6, 7, 8, 9, 0}

Since the thousand's place cannot be zero, available digits to fill the thousand's place = 9

Number of ways of filling the ten's digit = 10

Similarly, number of ways of filling the unit's digit = 10

$\therefore$ Total number of three digit numbers $=9 \times 10 \times 10=900$