How to architect Service Broker with three or more systems?

I'm trying to implement a Service Broker architecture for a business process involving three systems and I want to do it in the spirit of using SB as designed.

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  System 'A' is an accounting system. Whenever an invoice is 'released' in that system, a SB conversation starts.

  
  


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  System 'O' is a internally-developed orchestration system. Think of it as morally equivalent to BizTalk.

  
  


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  System 'C' is a database that has information that needs to be included on the final invoice to be sent to the customer.

  
  

How do I structure a conversation or series of conversations to touch all three systems?

Can I have 'A' submit to 'O', which does a lookup into 'C' - all in the context of a single conversation?