Find the Key of the Numbers

You are given three positive integers num1, num2, and num3.

First, convert each number into a 4-digit string.
If a number has fewer than 4 digits, add leading zeros to make it 4 digits long.

Then, form a new 4-digit result where the i-th digit is the minimum digit among the i-th digits of the three converted numbers.

Finally, return the resulting number without leading zeros. If the whole number becomes 0, return 0.