add_filter('dynamic_sidebar_params', 'add_classes_to__widget');
function add_classes_to__widget( $params ){
    $all_sidebars = wp_get_sidebars_widgets(); // get an array of all the sidebars
    $footer_sidebar = $all_sidebars['footer_sidebar']; // you need to know which sidebar you're getting: use its ID to select it.
    // this creates an array of the widgets shown in that sidebar, in order.

    // find the position of the widget ID in the array:
    $array_position = array_search ( $params[0]['widget_id'], $footer_sidebar );
    // so for the widget we're working on, its position is now in $array_position where 0 = first widget.

    if ( 2 === $array_position ){ // in this case the THIRD widget is smaller...
        $classe_to_add = 'col-2 '; // make sure you leave a space at the end
        $classe_to_add = 'class="'.$classe_to_add;
        $params[0]['before_widget'] = str_replace( 'class="', $classe_to_add,$params[0]['before_widget'] );
    } elseif ( 3 === $array_position ) { // and the FOURTH widget is larger...
        $classe_to_add = 'col-4 '; // make sure you leave a space at the end
        $classe_to_add = 'class="'.$classe_to_add;
        $params[0]['before_widget'] = str_replace( 'class="', $classe_to_add,$params[0]['before_widget'] );
    } else { // and all other widgets are normal.
        $classe_to_add = 'col-3 '; // make sure you leave a space at the end
        $classe_to_add = 'class="'.$classe_to_add;
        $params[0]['before_widget'] = str_replace( 'class="', $classe_to_add,$params[0]['before_widget'] );
    }
    return $params;
}