I'm about to partition my ext. hdd, in 2 parts. The first part will be MacOSX formated. I want the second partition to read/write both Ubuntu and Windows. Will NTFS do it for me?
You cannot install Ubuntu onto an NTFS partition, as NTFS does not support permissions.
But if you are just storing data, NTFS is fine and both Ubuntu and Windows can read/write from/to it.
Just in case you want something that is supported by all three platforms, check out the UDF filesystem.
EDIT: As mentioned by user Fabby, this answer details how you can actually achieve running Ubuntu and Windows on the same NTFS partition. I absolutely would not recommend it though, unless you really know what you are doing.