How to get subcategories of specific parent category?

You will have to add below dependency to your class :\Magento\Catalog\Model\ResourceModel\Category\CollectionFactory.

Code will be like this

use Magento\Catalog\Model\CategoryRepository;
....
class 
....
protected $categoryCollectionFactory;

public function __construct(
    ...
    CategoryRepository $categoryRepository
    ...
) {
    ...
    $this->categoryRepository = $categoryRepository;
    ...
}
public function getSubcategories($category, $levels = 2)
{
    if ((int)$levels < 1) {
    $parent_category_id = 3;
    $categoryObj = $this->categoryRepository->get($parent_category_id);
    $subcategories = $categoryObj->getChildrenCategories();
    foreach($subcategories as $subcategorie) {
        $response[] = [
            'name' => $subcategorie->getName(),
            'url' => $subcategorie->getUrl()
        ];

    }
   	return $response;
}