I have created a site that has several sub sites off it. eg group 1. group 2. group 3. When a user from group 3 logs in all he see's is group 3 site and group 2 user only see's group 2's site etc. But All 3 groups share certain documents, wiki sites etc. Is it possible to share these with all 3 sites but only having to maintain in the 1 site. eg I upload a document once and all 3 sites have access to it. Instead of having to upload the document 3 times to all 3 sites.
Which version of SharePoint are you using? sharepoint-15 isn't a valid version.
There are a few ways you can handle this. You can include the shared content at the top level site or a shared site that all three have access to and then use a Content Query Web Part (CQWP) or DataView Web Part (DVWP) to display the content on the sub-sites.
Sounds like you need to setup a few security groups with the relevant users in it and give those groups the appropriate rights on the shared resources.
You can then add links to the shared resources on each site's Quick Access bar using the Site Actions / Site Settings menu.