You need 3 USB sticks; one each for OS X, Linux, Windows.
- BACK UP YOUR SHIT FIRST
- start with a fresh install of OS X
- initiate bootcamp; create USB from Windows 10 ISO + download Apple drivers
- set bootcamp partition to total size for windows + linux
- intercept windows installer by restarting with option key held down, boot back into OS X
- open disk utility
- reformat bootcamp partition as mac partition (so it can be resized)
- decrease size of bootcamp partition to leave space at end for Linux
- reformat bootcamp partition as FAT (might not be necessary, i did it anyway)
- download refind from sourceforge and unzip it. do not install.
- download ubuntu ISO and dd it onto a USB stick
- reboot, hold option key, select EFI boot
- enter Ubuntu installer
- when partitioning, remove the last partition created in step 7 (NOT the bootcamp partition, the one at the end)
- add 2 new partitions - swap (8-16GB or so, use as swap) and root (all remaining space, format as Ext4, mount at /)
- install Ubuntu. leave bootloader set to install on /dev/sda... not sure if this is right but it doesn't break anything
- after Ubuntu installs, insert Windows install USB, reboot holding down Option, select Windows
- install Windows; when it reboots it might go back into Linux or OS X automatically... don't worry
- reboot machine into OS X installer USB, start Terminal:
`$ cd /Volumes/OS\ X/Users/jason/Downloads/refind`
`$ csrutil disable`
`$ ./refind-install`
- Remove all USB sticks and reboot!