Du måste använda råa uttryck som det :
$products = DB::table('products')
->join('hasDiscount', 'products.idProd', '=', 'hasDiscount.idProd')
->join('discounts', 'discounts.idDiscount', '=', 'hasDiscount.idDiscount')
->select(DB::raw('products.*,(products.price * discounts.tauxDiscount/100) as newPrice'))
->get();
https://laravel.com/docs/5.4/queries#raw-expressions