Multisite capabilities & roles

Hello.
I am sure that this has been posted already.
I am having a mystic vision. This is:

I run multi-site latest version. I am the super admin.
I have two administrators, they own their sites: two administrators, two sites. The sites are:

mysite.com/user1
mysite.com/user2

User 3 comes along and decides to sign up for a site, whilst browsing mysite.om/user1

Does User 3 get their own site (mysite.com/user3) or do they get User 1 sub-sub site, being mysite.com/user1/user3

Thank you for your reply.