Make sure your USB HDD is formatted in FAT32 (google for GUIFormat if you don't have that already)
Then you create a file named "launch.ini" containing this line:
default=usb:\F3\Default.xex
Next, download the latest F3 from here (you don't need the iso!) and extract it so that you have a default.xex i "F3" on your usb harddrive
Now you can boot your console with this harddrive connected and it should launch F3 for you =)