How can I make a combination array which contains maximum length?

I have these three arrays:

$arr1 = ['one', 'two', 'three'];
$arr2 = ['three', 'four'];
$arr3 = ['two', 'five', 'six', 'seven'];

And this is expected result:

/* Array
   (
       [0] => one
       [1] => two
       [3] => three
       [4] => four
       [5] => five
       [6] => six
       [7] => seven
   )

Here is my solution which doesn't work as expected:

print_r( array_unique( $arr1 + $arr2 + $arr3) );
/* Array
   (
       [0] => one
       [1] => two
       [2] => three
       [3] => seven
   )

How can I do that?