I like the sync control option as well. You can do some clever stuff with it.
However the term 'chain' suggests he may be referencing the Appworx product, and not Automic Workload Automation?
I do not recommend sync for your use case; https://docs.automic.com/documentation/webhelp/english/ALL/components/DOCU/12.3/Automic%20Automation%20Guides/help.htm#AWA/Objects/obj_Sync.htm?TocPath=Using%257CDesigning%257CObject-Specific%2520Properties%257CSync%2520(SYNC)%257C_____0
As I understand it, all that you want to do is to have JobA run with 3 slots from ChainB, and unlimited slots from ChainC, and you would like to accomplish this with two different queues.
Jared and I have already posted simple solutions to accomplish this.
I've also noticed that workflow task precondition rules have a "change queue" action available which provides another very simple solution.
Another solution is to clone JobA so that you have two of them; JobA.3 and JobA.unlimited. But then you would have two different sets of run history which may or may not be desirable.