Creating interface objects in the IR

I want to create a scenario in which I send a file from a Legacy system to the XI, process the message using the BPM and send it to SAP R/3.

In the integration repository I have 3 software components (one for each system: legacy, SAP APPL, SAP BASIS)

Where am I supposed to create the interface objects ? (AM I supposed to create all the objects under one namespace in one software component or maybe create in each software component only the relevant objects ?!?!)

The problem is that I always get to a point where I need to use objects that I defined in other software component... (and are not reachable)