biztalk: process unrelated messages in a given order

I are receiving messages from an external system.
There are three messagetypes, each representing a type of entity, and each message will contain a verb (create,update,delete)
This is represented as below.

Entity A (CUD)
Entity B (CUD)
Entity C (U)

Entity B have a relationship to entity A meaning that entity A must be created before entity B can be created/updated.
Enitity C have a relationship to entity B meaning that entity B must be created before entity C can be updated.

The messages will be used to update two other external systems.
The messages may be received out of order meaning that entity B may be received before entity A.

I am struggling to find a good pattern to ensure that messageentities are processed in correct order.
I would prefer not to use database or file system if possible.
Any suggestions as to how to solve this are more than welcome.