Okay, I see now. We plan a tool for that, but not available now.
Meanwhile you could try the following, though it maybe not quite convenient
1. Make target site holder Client 2 / Subscription 2 / domain3a
2. Websites & Domains > Advanced > Website copying > choose original site (domain3) > choose target site (domain3a)
3. Remove domain3. Rename domain3a to domain3
It will not move mail unfortunately.
BTW, what is a purpose of such relocation? Is it like one account is a web designer and another one is a customer?
Regards