Multiple sites linked to one payment in Pro Sites

Hey guys, is it possible to do this? For example, a customer signs up for one paid site, and I want to give them 2 more sites for free. If they cancel one of them, all the sites should be cancelled.

That way they can't just cancel the paid site and keep the 2 free ones.

The only way I see to do this is to link multiple sites to one paid site. Is that possible?