my $sort =     
    sub { 
        my $rhash = shift;
        return sort { 
            $rhash->{$a}{'field3'} cmp $rhash->{$b}{'field3'} 
            } keys %{$rhash}
    };
    $hash_out = show_these($hash_ref,[ 'field1','field2','field3'  ],$sort);