Your ServerName/ServerAlias directives are wrong. ServerName should be hostname. You probably should just delete ServiceAlias.
Then just do the obvious and duplicate VirtualHost/Listen directives, just changing the port number and locations of scripts in the file system.
Finally, do not set DocumentRoot to be where your Django code is as it makes it easier to accidentally expose your source code to download if you stuff up Apache configuration. So, just remove DocumentRoot directive from VirtualHost for Django sites.
Listen 80
<VirtualHost *:80>
ServerName www.example.com
WSGIScriptAlias / /opt/django/site1/apache/django.wsgi
Alias /media /opt/django/site1/media/statics
Alias /admin_media /home/myuser/Django-1.1/django/contrib/admin/media
<Directory opt/django/site1/apache>
Order allow,deny
Allow from all
</Directory>
<Directory /home/myuser/Django-1.1/django/contrib/admin/media>
Order allow,deny
Allow from all
</Directory>
</VirtualHost>
Listen 8080
<VirtualHost *:8080>
ServerName www.example.com
WSGIScriptAlias / /opt/django/site2/apache/django.wsgi
Alias /media /opt/django/site2/media/statics
Alias /admin_media /home/myuser/Django-1.1/django/contrib/admin/media
<Directory opt/django/site2/apache>
Order allow,deny
Allow from all
</Directory>
<Directory /home/myuser/Django-1.1/django/contrib/admin/media>
Order allow,deny
Allow from all
</Directory>
</VirtualHost>
Listen 8090
<VirtualHost *:8090>
ServerName www.example.com
WSGIScriptAlias / /opt/django/site3/apache/django.wsgi
Alias /media /opt/django/site3/media/statics
Alias /admin_media /home/myuser/Django-1.1/django/contrib/admin/media
<Directory opt/django/site3/apache>
Order allow,deny
Allow from all
</Directory>
<Directory /home/myuser/Django-1.1/django/contrib/admin/media>
Order allow,deny
Allow from all
</Directory>
</VirtualHost>
I have also add missing Directory directive for allowing access to static files. You should review paths however.
Make sure you read:
http://code.google.com/p/modwsgi/wiki/IntegrationWithDjango
http://code.google.com/p/modwsgi/wiki/ConfigurationGuidelines#Hosting_Of_Static_Files
for further information.
UPDATE 1
BTW, since you are using PHP in same Apache, you would be much better off using mod_wsgi daemon mode and push each Django instance out into its own separate process. That allows those processes to be multithreaded, even though main Apache processes are forced to be single threaded because of PHP. End result will be much much less memory being used than if running multiple Django instances in each process under embedded mode with prefork MPM. Your Django code just needs to be thread safe. Configuration in addition to above would be to add WSGIDaemonProcess/WSGIProcessGroup to each Django VirtualHost, where name of daemon process group is different for each VirtualHost.
<VirtualHost *:80>
WSGIDaemonProcess site1 display-name=%{GROUP}
WSGIProcessGroup site1
... existing stuff
</VrtualHost>
<VirtualHost *:8080>
WSGIDaemonProcess site2 display-name=%{GROUP}
WSGIProcessGroup site2
... existing stuff
</VrtualHost>
<VirtualHost *:8090>
WSGIDaemonProcess site3 display-name=%{GROUP}
WSGIProcessGroup site3
... existing stuff
</VrtualHost>
This also allows you to more easily restart each Django instance without restart whole of Apache. Read:
http://code.google.com/p/modwsgi/wiki/QuickConfigurationGuide#Delegation_To_Daemon_Process
http://code.google.com/p/modwsgi/wiki/ReloadingSourceCode