How to read external USB hard drive formatted ext3 from Windows 7?

EXT2FSD works great for me on Windows 7 X64. http://www.ext2fsd.com/

You can try Ext2 IFS for Windows, which may require a bit of tweaking on 7. Another option is to use Cooperative Linux within Windows itself to access the volumes.

I would recommend using Total Commander with DiskInternals Reader Plugin! Easy and powerful!