How to triple boot a 2015 Macbook Pro:

(with OSX 10.10 Yosemite, Windows 10, and Ubuntu 14.04 LTS)

In OSX, remove the Core Storage designation from your OSX partition. Instructions here.

Launch OSX Bootcamp. Create a Windows installation USB drive (I've done Windows 8 and 10) and create a hard drive partition for Windows.

When the computer reboots into the Bootcamp Windows installer, hold down the power button to shut down, then press Option+Power to restart into the Mac bootloader. Select and boot the OSX partition.

In OSX, run Disk Utility. Reduce the size of the OSX partition, then create a new partition for Ubuntu in the empty space. File format on the Ubuntu partition shouldn't matter at this point. I used FAT.

Create an Ubuntu installation disk whatever way you prefer with (I believe) whatever version you prefer.

Shutdown again and Option+Power restart to get to the Mac bootloader, then select the Ubuntu install disk (should be labeled EFI, there might be two of them, either should work). Reformat the partition you created in Step 4 for Ubuntu and complete the installation process, then shutdown. (If you reboot into the Mac bootloader at this point, Ubuntu should not be visible)

Power (not Option+Power) restart. Your Macbook should boot into the Bootcamp installation partition. Complete the Windows installation process normally. The Macbook will reboot several times. If ever it reboots into the wrong partition (it shouldn't), then manually shutdown, Option+Power boot, and select the Bootcamp partition manually.

Almost done. After Windows is fully installed and you can use it normally, Option+Power reboot into OSX, then install rEFInd.

Shutdown and perform a regular boot. You will arrive at the rEFInd menu with a 20-second timer, and all three OSs should be visible.





Update:

I have had the problem, twice now, that whenever I install OSX updates, rEFInd disappears. When I power on the computer, it boots straight to OSX. This problem is fixed by simply reinstalling rEFInd from within OSX. Back to normal.



More Update:

Some users might receive errors when returning to the Windows installation process. Julian Leland recommends clearing the hybrid MBR data from within OSX via Enjoy.I have had the problem, twice now, that whenever I install OSX updates, rEFInd disappears. When I power on the computer, it boots straight to OSX. This problem is fixed by simply reinstalling rEFInd from within OSX. Back to normal.Some users might receive errors when returning to the Windows installation process. Julian Leland recommends clearing the hybrid MBR data from within OSX via this process

I've written about triple booting Macbooks before, but the process has changed recently. I bought a Macbook Pro a while back which runs OSX 10.10 Yosemite out of the box, and that introduces a few new hurdles. I found a few detailed instructions around the web on how to run Yosemite, Windows 8, and Ubuntu on one machine, but they all involve significant terminal work and partition juggling, and I was looking for something a bit more straight forward, a bit cleaner. I was pretty sure there must be an easy way to install all three on a single Macbook, and I was right. It's simple.The combination of Yosemite and the 2015 hardware introduce two new problems: one, Yosemite doesn't think users should be allowed to resize partitions manually and, two, the 2015 Macbook's bootloader doesn't seem to recognize Ubuntu at all. Luckily, both of these are easy to fix. Yosemite declares the OSX partition to be "core storage" (I don't even know what that means), but that designation can be removed easily. As for the boot loader, the rEFInd bootloader (a spin-off of rEFIt) does recognize Ubuntu. Windows still works best when installed via Bootcamp, and this requires that it be installed both first and last, but we can get around that too. Basically, we start the Bootcamp process, then install Ubuntu, then finish the Bootcamp Windows installation. The whole process takes a few hours, but it works perfectly. When I boot my Macbook now I get the rEFInd loader screen with three icons.