#!/usr/bin/perl

# http://perlmonks.org/?node_id=1196143

use strict;
use warnings;

$_ = join '', sort <DATA>;

1 while s/^(\d+,\d+,)\K (\S+)\n\1(\S+)/ $2 + $3 /emx;

print;

__DATA__
3211111,100,3.2
3211112,101,3.2
3211111,100,1.2
3211112,100,2.2
3211113,100,5.2
3211112,100,0.3