I have 2 tables A and B both with a composite primary key consisting of 2 columns,

My question is how would a .hbm.xml file look like for specifying a third table which has a primary key consisting of 3 columns, column 1,2 are a foreign key to table A and column 2 and 3 are a foreign key to table B.

Only if you have the time :)

Bz

Just do the mapping as if you would have 4 columns. But you have to set the update and insert attributes to false.

Now you're left with the problem of how to write to the table.

I've solved a similar problem by defining ordinary properties for each of the foreigen key columns. I call them mirror properties.
In the onSave and onFlushDirty methods of an Interceptor you could detect, whether there are references between the Objects, if yes update the DB by setting the appropriate mirror properties.

HTH
Ernst