Your Windows 7 partition is now NTFS, you can't shrink it in Mac OS, and if your shrink it in Windows 7, it may corrupt your GUID. You can shrink MacOS partition in Disk Utility, then install Ubuntu following a triple boot guide. The Ubuntu boot choice will appear beside Mac OS and Windows.
There is an easier way to install Ubuntu that you don't have to mess up your current HDD. First, set the Windows 7 partition to active then installing Ubuntu in WINDOWS (Wubi), restart, then set the MacOS X drive to active. When you boot, you will only see Mac OSX and Windows, choose Windows and you can select booting to either Windows or Ubuntu.
P/S: I installed Linux Mint and Jolicloud using the first choice, after sever months, I can't boot to Linux anymore. It only shows : grub rescue.