I'm running a website with 3 domains redirecting to that domain (without a index file, just by admin panel at the provider). Is there a way to SEO these redirecting domains? When I was thinking about this I realized that by redirecting you probably can't upload a .htaccess because you don't have webspace.
I think it's not possible, but i ask is just to be sure.
You're right - it's not possible to SEO a domain that simply redirects to a different domain.
Meta tags / data won't get you any SEO benefit anyway - it's all about content and authority.