I set up the triple boot with these operating systems on my iMac last Sunday after trying this a few billion times over the previous week.
It took some research and alot of trial and error to figure it out, so I am sharing my experience so that maybe it can save you the troubles of figuring it out. The thing is that Windows and Haiku don’t understand GPT (at least not all that very well) and all the three operating systems don’t see eye to eye with each other regarding the partition usage and definitions.

The next four paragraphs are general theory about the issues you can run into. You can skip them if you want to go straight to the installation part.

Mac OS X is like an old by-the-book government employee. It would only recognise the partitions that declare themselves to contain the file systems that it understands, so a partition containing BFS is considered to be free space. You can get into trouble if you try to delete a BFS partition through Mac Disk Utility.

Windows 7 is the monster screaming All-your-partitions-are-belong-to-us on you and your other partitions. Its installer would overwrite the partition containing the Haiku installation with its boot files because it will be marked as NTFS if you create it from the Disk Utility or GParted.

Haiku is the new-kid-on-the-block who is bright but is too inexperienced to get things right. It cannot handle the GPT and its partitions are recognised as NTFS with Windows Installer, free space by Mac Disk Utility and will usually be deleted if you use the Refit’s disk utility to sync your GPT and MBR.

So the partition job should be done entirely by GParted the only sane voice that handles the GPT partitions better. And Window 7 needs to be installed before Haiku so that it doesn’t overwrite Haiku’s partition.

I have explained these steps below:

1. Install Refit and restart your computer two times for the Refit Boot menu to come up. We do NOT use boot camp for triple boot set up.
2. Boot from an Ubuntu livecd or the Gparted boot cd. I used Ubuntu livecd because Gparted could not figure out its boot configuration on the iMac.
3. Run Parted/Gparted to create an NTFS partition for your Windows Installer.
4. Use the rEFIt Partitioning Tool to sync the GPT and MBR tables otherwise your new partition may not show in your Windows Installer.
5. Use Windows installer to format this partition and install Windows into it.
6. Restart the computer after the installation and run Parted again.
7. Create another partition for Haiku.
8. Use the rEFIt Partitioning Tool to sync the GPT and MBR tables
9. Install Haiku into this new partition.

Your triple boot should now work fine.
cheers,
Dhruwat aka Unitedroad