You may use the following hook to re-order the fields in your cart. Replace 12 with your form ID. Replace 123, 234, and 345 with your field IDs. 123 will be first, 234 will be second, and 345 will be last.

add_filter( 'wc_fp_cart_fields', 'frm_reorder_wc_cart_fields', 10, 2 );
function frm_reorder_wc_cart_fields( $fields, $form_id ) {
    if ( $form_id == 12 ) {
        $reordered_fields = array();
        foreach ( $fields as $key => $field ) {
            if ( $field->id == 123 ) {
                $reordered_fields[0] = $field;
            } else if ( $field->id == 234 ) {
                $reordered_fields[1] = $field;
            } else if ( $field->id == 345 ) {
                $reordered_fields[2] = $field;
            }
        }

        $fields = $reordered_fields;
    }

    return $fields;
}