Change the third comma with : regex

I want to change the third comma in this string using regex in notepad ++

145, 45, 67, 688

With a ":"
Like this :

145, 45, 67: 688

I tried this

(?:,)(,)*

I think I am missing something

>Solution :

If the third comma is always the last, you can check for the end of the string/line with $:

,(?=[^,]*$)

Demo: https://regex101.com/r/c0XANJ/1