Problem: Largest 3-Repeated-Digit Number in a String

You are given a string num that represents a large number. A good number is defined as:

1. A substring with a length of exactly 3 characters.
2. All characters in this substring are the same digit.

Your task: Return the largest good number as a string. If there is no such substring, return "" (an empty string).

Additional Notes:

• A substring is a sequence of consecutive characters within the string.
• Leading zeroes are allowed in a good number.

Examples:

Example 1
Input: num = "6777133339"
Output: "777"

Example 2
Input: num = "2300019"
Output: "000"

Example 3
Input: num = "42352338"
Output: ""

Constraints:

• 3 <= num.length <= 1000
• num consists only of digits 0-9