1. The problem asks: If the decimal moves three places to the left, which operation does that represent and why? 2. When the decimal point moves to the left, the value of the number decreases by a factor of 10 for each place it moves. 3. Moving the decimal three places to the left means dividing the number by $10^3 = 1000$. 4. Therefore, the operation is division, because dividing by 1000 shifts the decimal three places left, making the number 1000 times smaller. 5. The correct choice is: "Division, because the decimal shifts left when you divide, making the number's value 1,000 times less."