How do i declare an attribute to be a part of the composite key?

I'm using the article http://boris.kirzner.info/blog/archives/2008/07/19/hibernate-annotations-the-many-to-many-association-with-composite-key/ as a reference to create a join table between 2 classes but i need to add another attribute to the composite key to avoid constraint issues.

I added another attribute in the join class that i want to be unique to the 2 composite keys but those keys can repeat. Since this data will be managed by me, the easiest way to solve this is to make all 3 attributes a composite key.