How to link several outside processing purchase orders

I want to automate a process where we open a production order for an item; do operation one, send operation two outside (using PP02 control key) to vendor A who when done will send it to vendor B who will do operation 3 and who when finished will send the material to vendor C who will do operation 4 and when done will return it to the plant who will finish the production order by doing operations 5 and 6.  How do I do this?

Thanks