use strict;
use warnings;

sub readfile {
    my ( $filename, $hashref, $headref ) = @_;
    open my $fh, "<", $filename or die "Cannot open $filename!\n";
    my $headers = <$fh>;
    my @h = split /\s/, $headers;
    $headref->{$_}++ for @h[3..$#h];
    while( <$fh> ) {
        my @line = split /\s/;
        $hashref->{$line[0]}{$line[1]}{$line[2]}{$h[$_]} = $line[$_] for 3..$#h;
    }
    close $fh;
}

my %joined;
my %headers;
for my $file ( qw/file1.txt file2.txt file3.txt/ ) {
    readfile( $file, \%joined, \%headers );
}

print "ID NAME date ", join( " ", sort keys %headers ), "\n";
for my $id ( sort keys %joined ) {
    for my $name ( sort keys %{$joined{$id}} ) {
        for my $date ( sort keys %{$joined{$id}{$name}} ) {
            print "$id $name $date ";
            print join " ", map { $joined{$id}{$name}{$date}{$_} // "--" } sort keys %headers;
            print "\n";
        }
    }
}