If I'm not wrong, you're installing more than Wordpress Plateforms on your main domain into sub domain.

1- As of your requirement, it's risky, messy, and not a good practice to allow non-registered users or guest to to post content without registration or permission. Yet, if you want to do so, could serve your requirement.

2- It's possible if you good at programming and know what you are doing but it's also a bad practice to install seperate Wordpress Systems on a single site. Even if you use the same User Info tables, each Wordpress still have different cookies. This an initial issue. Other than that it'll make your site extremely using many resources of the server. I'd recommend to use Wordpress Multi-Site Version instead or using only one Wordpress with seperates sections/themes % redirect to sub domain, but it's still needing more savy codes.

You might consider one install of WordpressMU, which is bundled as part of
Wordpress now. You would set it up to install new Wordpress blogs
Subdomains, just like Wordpress.com does.

It's time consuming, but will be a helluva lot easier, and less time consuming
that trying to integrate three different user stacks through one login.

Special considerations: You're going to have to be on a host that will let you
build subdomains on the fliy. Not all will, so ask before you go through all this
trouble.

1. Make a complete backup of the domain and all the subdomains. We're
going to be getting the WP-content folders in a little while. Again keep watch
of labels so you know which is which.

2. Go to the Wordpress Dashboard to each of your blogs.

3. Go to tools > Export. And export an XML copy of each blog. Make sure
you label them so that you know which domain each goes with.


4. Take down the primary domain first and install a brand new copy of
Wordpress. Follow these instructions to set up a proper multisite, network.
Create A Network « WordPress Codex

The following directions you will setup for each of your blogs.

5. Go to the zip download of whichever blog your working on, and navigate
to the the wp-contents folder.

6. Log into the root of the primary domain.

7. Upload this wp-contents folder of the one that's there now.

8. Log into Wordpress and navigate to Dashboard > Tools > Import the XML
backup with attachments, and you should now have your old blog back, now
with multisite capabilities and single login capabilities.

9. Repeat the above steps with each subsequent blog, until you get back to
square one.

19, This isn't cut and dry. There will be testing, regardless of the options you use,
but depending on you tech level you may be able to pull this one off wihtout
hiring someone.

Good luck.