Educated guess: (The key was to merge the arrays by using `array_merge()`)

    // Put all values in an array().
    $AllItems = array_merge($FruitsCategory, $DrinksCategory, $DrinksCategory); // use array_merge
    
    foreach ($AllItems as $Item) {
      $article->field_article_all_items[] = ['target_id' => $Item];
    }
    
    $article->save();

Also it can be used within the node creation:

    // Put all values in an array().
        $AllItems = array_merge($FruitsCategory,$VegetablesCategory,$DrinksCategory);
    
    // Create a new "Article" node.
        $article = \Drupal\node\Entity\Node::create([
            'type' => 'article',        
            'title' => Test,
            'langcode' => 'en',
            'uid' => '1',
            'status' => '1',
            'created' => $current_time,
            'field_article_all_items' => $AllItems,  // use array_merge
            ]);
    // Save the created node.
        $article->save();