How can I format an external HDD as EXT3?

As far as I know, there is no format EXT3 - there are formats for Windows and Mac OS Extended (Journaled) for Macs.

So what type of format is this supposed to be?

Or, are you possibly asking about renaming it? If so, simply highlight the icon, hit the return key and the name will change into an editable field. Type the name you want and hit return again.

Mac OS X does not support Linux file systems, such as ext3.

It might be possible to install Linux in virtual machine if you really must do this, or boot you Mac with a Linux live CD/DVD.

Thanks! Never having worked with Linux, had absolutely no idea that actually existed.......