THE DON’TS WHEN YOU HAVE A 3RD LEVEL AND SECOND LEVEL DOMAIN
Current best practices recommend against having duplicate versions of the same website with different domains, so if that’s what you were thinking—stop right there.
You’ll get the most benefit for domains that either permanently redirect to your main URL (called a 301 redirect) or if the content for each domain is unique.
No matter what, don’t duplicate your website under a different domain
Perhaps the worst option for using multiple domains is to duplicate your exact website and point to a different domain name. Search engine algorithms are smarter than ever, and they will not value duplicate content.
Furthermore, duplicate content can also negatively impact your domain authority and general credibility with users and other websites. Instead, the best way to boost your SEO is to get other sites to backlink to your website.
If you are considering a custom domain for marketing purposes or to attract more users, determine if you’ll redirect your domain back to the main site or if you have enough unique content to support an entirely separate website.