Jitsu doesn't seem to support multilevel subdomains when deploying an app.
For example I have app 1 with:
And I wish to have app 3 with
However it tells me that the subdomain is in use (presumibly because it only looks at blah.blah2
The intent is for only the root subdomain to be taken by users, thus I can do this without issue:
This is because I own the "duck" subdomain. In your case it seems blah3 has been taken by another user... If you could give me concrete example of the issue you are seeing I'd be happy to investigate further.