The first part is quite easy using the shared users table (1.4 onwards). Set up the first site as normal. Then on the second site, before installation set a different table prefix in config, and set the option QA_MYSQL_USERS_PREFIX to point to your users table on the first installation. Then run the installation. Repeat for the other two sites.
For the latter part, I'm not sure what the best method would be. The easiest would be a custom PHP script that does a regular SQL query and selects questions from all the databases.
If you want the same Q&A layout then maybe you could set it up as a normal Q2A site, block all question asking on that site, but write a plugin that grabs questions from all databases and links them to the subdomains.
P.S. it's funny how you managed to misspell "subdomain" 5 times with completely different misspellings every time ;)