I think the easiest way to create a new partition is using GParted Live
Just download it, burn it onto a cd and boot from that.
The usage of the program then is quite self-explanatory; on the website you can find all the documentation, but even just a look to the screenshot
section may be useful.
Basically you have to squeeze the partitions you are actually using for MacOSX and Win7 to free up the space needed to create the new ext3 partition dedicated to Ubuntu.
I hope to be have been helpful!
p.s.: remember to make a full backup of all your data and configuration files before making any change to the partition table.