title: How to find and replace before the match?
body: <p>I have comma delimited data like 12345,6789,567890. What i would like is to find 3 digits before the first comma and put a comma where they start. Given the above entry, i want to find 3 digits before first comma, that would be 345 and transform the entry "12345" to "12,345". </p>
  
  <p>What is the regular expression to find 3 digits before first comma occurrence and how to put comma before those 3 digits. I am using vim.</p>