show_these($hash_ref,[ field3, field2, field1 ]);

sub show_these {
    my ($dr,$show) = @_;
    my $output;
    for my $row (keys %{$dr}) {
        for (@{$show}) {
            next if !$dr->{$row}{$_};
            $output .= qq!$_ = $dr->{$row}{$_} !;
        }
        $output .= "\n";
    }        
    return $output;
}