use warnings;
use strict;

use Data::Dumper;

my $num_hashes = 3;

my %h1 = (
    a => 1,
    b => 2,
    c => 3,
);

my %h2 = (
    a => 4,
    b => 5,
    d => 10,
);

my %h3 = (
    x => 20,
    p => 15,
    b => 6,
    a => 12,
);

my %key_list;

for my $hash (\%h1, \%h2, \%h3){
    $key_list{$_}++ for keys %$hash;
}

my %combined;

for my $hash (\%h1, \%h2, \%h3){
    for (keys %key_list){
        next if $key_list{$_} < $num_hashes;
        push @{ $combined{$_} }, $hash->{$_};
    }
}

print Dumper \%combined;

##</code><code>##

$VAR1 = {
          'b' => [
                   2,
                   5,
                   6
                 ],
          'a' => [
                   1,
                   4,
                   12
                 ]
        };