Hi,
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!
cheers
Mattia
p.s.: remember to make a full backup of all your data and configuration files before making any change to the partition table.
