>perl -wMstrict -le
"my $data = q{'A','B','C','D','E','F','G','H','I','J','K','L','M'};
 ;;
 my @ra = split m{ (?: , [^,]+){3} \K , }xms, $data;
 ;;
 printf qq{:$_:  } for @ra;
"
:'A','B','C','D':  :'E','F','G','H':  :'I','J','K','L':  :'M':