I have had issues in the past when adding an extra partition after my OS X and Bootcamp partitions, where it would break Bootcamp. I want to add a third bootable partition with Fedora 21, what is doable without reinstalling OS X/Bootcamp first?
I don't think it's possible to have a third partition with Win 7. You need Win 8.1 or the new Win 10 preview for it to work.
I am triple booting Yosemite with Win 10 preview and Linux Mint.
Actually it is possible as the limit is for MBR not GPT and they need not match. You can have 100 GPT partitions if you want as long as you define a protective MBR partition covering partitions partitions 4-100. You'll then only be able to see the 1st 3 partitions in Windows but can still boot any of them. Alternatively you can make your MBR stop after the 4th GPT partition and the rest will show as unallocated space in Windows.Also not possible if Windows 8.1 and 10 are on Ivy Bridge or earlier Macs.
Ivy Bridge and earlier Macs do not support the UEFI 2.0 standard. Only Haswell and later Macs support UEFI 2.0, and henceforth Windows in native UEFI.
On Ivy Bridge and earlier Macs, Boot Camp will use hybrid GPT-MBR, causing the 4 partition limitation to occur. On Haswell and later Macs, Boot Camp will use pure GPT.