-----Description-----  
This task requires writing a Lean 4 method that takes a given string and returns a new string where every occurrence of a space, comma, or dot is replaced with a colon. The transformation must preserve the original string’s length and leave all other characters unmodified.

-----Input-----  
The input consists of:  
s: A string.

-----Output-----  
The output is a string:  
- The returned string must have the same length as the input string.  
- Every space, comma, or dot in the input string is replaced with a colon.  
- All other characters remain unchanged.

-----Note-----  
There are no preconditions; the input string is assumed to be non-null.