... \s*([^,]+)\s*,...
Find some whitespace, or perhaps none at all.
Find anything that is not a comma, at least once, 
which *includes* whitespace, then...
Find some more whitespace, perhaps none
Find a comma

##</code><code>##

m#(?:[^,]*,\s*){3}(.*?)\s*,#

##</code><code>##

(?:        ## Group, but do not store it into $1
[^,]*      ## Anything that is not a comma
,          ## Followed by a comma
\s*        ## Followed by possible whitespace
){3}       ## Find three of these (the first three)
(.*?)      ## Match any character, but don't be so greedy about it
\s*        ## Possible whitespace
,          ## Stops at first comma, because we are not being greedy