Let me start by saying thank you for this post and all the helpfull information. After 7 attempts I have my tripple boot Mac running great.

These are the items that hung me up and what steps I changed a bit to resolve them. I only had trouble with the Linux install and the windows MBR getting corrupted.

First: On my Macbook the hotkey to get to a command line / Terminal was "FN+CTRL+ALT (OPTION)+F2"

Second: I am new to Linux so I did not know to use "sudo" in front of the command line for backing up and replacing the mbr so I changed

dd if=/dev/sda of=/tmp/sda.mbr bs=512 count=1

to "sudo dd if=/dev/sda of=/tmp/sda.mbr bs=512 count=1"

Third: The replacement of the MBR part did not work for me to apply at any point before Grub ran its part of the process. I am not sure if the original poster did this from command line or if something was different but the Grub bootloader pops up almost immediately after the Partitioner.

As an additional note to select where Grub puts the bootloader you need to select Advanced Options on the final install screen immediately after inputting your name and password.

So for the windows bootloader to not be corrupted I had to restore the MBR at the very end of the installation when the installation asks for a restart. It is at that point that I ran the command again using sudo,

"sudo dd if=/tmp/sda.mbr of=/dev/sda"

And my last note is that it is "FN+CTRL+ALT (OPTION)+F7" to exit back to the installation.

Thanks again for the post and I am enjoying my tripple boot Macbook!