The best and the most certain is to install Grub2(Linux) last then you can add the Mac's entry later in boot/grub/grub.cfg file through etc/grub/custom.It was the only way for me to triple boot successfully.

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System specs: Desktop PC,
Nvidia GeForce 210,1GB,GDDR3,PCI-E 16x gpu,
6GB Ram(4x2GB),DDR3,1333mhz,
Ubuntu 12.10 x86_64/Windows 8 Pro x64/Mountain Lion 10.8.2,
Intel Core i3 3.1Ghz, cpu,
Gigabyte H61MA-D3V motherboard.