Hello Amanda,
The way to accomplish this would be to utilize “\K”.
Here is the GREP I used to make it work when testing:
.+(?=,.+,.+$)\K.+
The first bit — .+ matches all characters that fall before the positive lookahead
The positive look ahead — (?=,.+,.+$) matches to a comma, 1 or more characters, another comma, 1 or more characters and then the end of the paragraph.
Now for the really nifty part. The “\K” is used to ignore everything that has been matched by the GREP before it. That means that all of the matched characters from the first part of the GREP string — .+(?=,.+,.+$) are essentially being used to tell the second part — .+ (match 1 or more characters) where to begin.
So the first part of the GREP string (the part before the “\K” actually matches all of the text in your paragraph up until the second to last comma, then we use \K to tell the GREP to apply the style to every character after that match.