I, also, don't understand the rule for positioning the  '-' (hyphen) filler character(s), but here's an approach. (Update: I guess I should also admit I don't really understand 'alignment' either as used in the context of the OP.) Maybe not the most efficient, but I'm hopelessly addicted to regexen.

c:\@Work\Perl\monks>perl -wMstrict -le
"use List::Util qw(max);
 use Data::Dump;
 ;;
 my @List1 = qw(a a b);
 my @List2 = qw(b b c);
 my @List3 = qw(c d d);
 ;;
 my @runs = do {
   my $cat = join '', @List1, @List2, @List3;
   my $s;
   grep $s = !$s, $cat =~ m{ ((.) \2*) }xmsg;
   };
 ;;
 my $longest = -1 + max map length, @runs;
 my @padding = ('-') x $longest;
 ;;
 my @padded = map [ (split(''), @padding)[0 .. $longest] ], @runs;
 dd \@padded;
"
[
  ["a", "a", "-"],
  ["b", "b", "b"],
  ["c", "c", "-"],
  ["d", "d", "-"],
]
[download]

Update: Here's another, slightly different approach using unpack; might be slightly faster.

c:\@Work\Perl\monks>perl -wMstrict -le
"use List::Util qw(max);
 use Data::Dump;
 ;;
 my @List1 = qw(a a b);
 my @List2 = qw(b b c);
 my @List3 = qw(c d d);
 ;;
 my @runs = do {
   my $cat = join '', @List1, @List2, @List3;
   my $s;
   grep $s = !$s, $cat =~ m{ ((.) \2*) }xmsg;
   };
 ;;
 my $max_pad = -1 + max map length, @runs;
 my $padding = '-' x $max_pad;
 ;;
 my @padded = map [ unpack qq{a (a)$max_pad}, $_ . $padding ], @runs;
 dd \@padded;
"
[
  ["a", "a", "-"],
  ["b", "b", "b"],
  ["c", "c", "-"],
  ["d", "d", "-"],
]
[download]

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