Final answer:
The probability that a number chosen randomly from the set {0, 1, 2, ....., 1000} is a three-digit number is 90%.
Step-by-step explanation:
To find the probability that the number chosen is a three-digit number, we need to find the number of three-digit numbers in the set {0, 1, 2, ....., 1000} and divide it by the total number of numbers in the set. The set contains 1000 numbers, out of which 900 are three-digit numbers (from 100 to 999). Therefore, the probability is 900/1000 = 0.9 or 90%.