My USB Technique
This is what I did to update my 2011 335D with the North American Nav DVDs.
1. I have a 32GB Kingston USB 2.0 Flash drive formatted as FAT32
2. I used UNETbootin in Linux to burn the first disk ISO to the flash drive. You need to do this to write the correct boot sector to the flash.
3. After that was complete, I opened the remaining two ISO discs in ubuntu archive manager and extracted the files directly to the flash drive in the correct directory.
4. That process took about 40 minutes.
5. I put the USB drive in the glove compartment and drove the car around for about an hour. It never asked for another DVD or USB and it worked perfectly.
Hope this helps.