Build and print a Hash of Arrayrefs like this:

my ($i,%x);
$i=0;
$x{$_}[$i++%3]++ for split //, $a;

for my $k (sort keys %x){
  my $aref = $x{$k};
  print "$k ";
  print for @$aref;
  print $_;

}
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(untested)