this worked for me
$category = new Category(8, (int)Context::getContext()->language->id);
$category1 = new Category(9, (int)Context::getContext()->language->id);
$category2 = new Category(10, (int)Context::getContext()->language->id);
$category3 = new Category(11, (int)Context::getContext()->language->id);
$nb = (int)(Configuration::get('HOME_FEATURED_NBR'));
$products = $category->getProducts((int)Context::getContext()->language->id, 1, ($nb ? $nb : 10));
$cat=$category->getProducts((int)Context::getContext()->language->id, 1, ($nb ? $nb : 8), "position");
$cat1=$category1->getProducts((int)Context::getContext()->language->id, 1, ($nb ? $nb : 8), "position");
$cat2=$category2->getProducts((int)Context::getContext()->language->id, 1, ($nb ? $nb : 8), "position");
$cat3=$category3->getProducts((int)Context::getContext()->language->id, 1, ($nb ? $nb : 8), "position");
$products = array_merge($cat,$cat1,$cat2,$cat3) ;
In the module i set to display 3 products, and on my homepage, it shows 12 products( 3 products x 4 categories selected)
Thank you Vekia