I'm using SSRS 2005 - I think this part of it works in the same way as 2008.
As far as I can tell, you can't have folders within projects, but you can have multiple projects within a solution.
To create a new folder, right-click on the solution in the Solution Explorer and select Add>New Project...
Type in your new Project Name (eg. MyProject), and select Report Server Project from the list of Visual Studio installed templates. Click on OK, and your new Project should appear at the end of the list of projects in the Solution Explorer.
(There are other ways of setting up a new Reports project, but this seems to be the quickest.)
If you now right-click on your new Report Project and select Properties, you can see the TargetReportFolder, which will default to your new Project Name (eg. MyProject). When you deploy reports from SSRS, they are deployed to this location. (You can change the location, if you wish - I find it easier to keep track of what's going where by using the Project name.)
You will need to copy any data sources to be used in each project, into the data sources folder of all projects that use that data source. By default, OverwriteDataSources is set to false, so when you deploy a new report, it will use the data source already deployed to the Report Manager environment.
So to get the Report Manager structure that you want to see:
- Create Projects called Folder A and Folder B
- Move/copy Report1 into the Reports folder in Project Folder A
- Move/copy Report2 and Report3 into the Reports folder in Project Folder B
- Move/copy data source DEV into the Shared Data Sources folders in Projects Folder A and Folder B
- Deploy your reports
Don't forget to check your changes into source control.