As far as I can tell this is working as designed.  When you have 3 upload fields like that, you get an error status for each one.  A print_r($_FILES) on the completion of the upload of a file upload form with 3 files and a text field gives me this:

Array
(
    [file] => Array
        (
            [name] => Array
                (
                    [0] => Screenshot_1.png
                    [1] => colourmap.pdf
                    [2] => 
                )

            [type] => Array
                (
                    [0] => image/png
                    [1] => application/pdf
                    [2] => 
                )

            [tmp_name] => Array
                (
                    [0] => /var/tmp/phpERCmvY
                    [1] => /var/tmp/phpDJfvBw
                    [2] => 
                )

            [error] => Array
                (
                    [0] => 0
                    [1] => 0
                    [2] => 4
                )

            [size] => Array
                (
                    [0] => 162434
                    [1] => 239943
                    [2] => 0
                )

        )

)

The error array has 0, 0 and 4.  That means the first 2 file upload fields uploaded fine and the 3rd got an error code 4 which means "no file uploaded".