You need to create a new branch for eXo Platform 3.5 extension project, then clone your source code of eXo Platform 3.0 with that of eXo Platform 3.5.

The way to package your project customization called extensions has been introduced in eXo Platform 3.0. Therefore, by using the WAR extension configuration approach, you can overload eXo Platform's default files defined in WARs, such as gtmpl, favicon, xml, jsp. For more details on how to create your extension project, refer to the Create Your Own Portal section.

Because there are not any changes on the eXo Platform 3.5 extension mechanism, your extensions should be mostly compatible. However, if your extension was overriding either of the eXo Platform 3.0 built-in files, you should check that the file has not been changed in eXo Platform 3.5 and update it if any.