How to 3-way sync?

[Nospampls]

My topic is too vague, but I don't know how to pose this question.  I have 3 computers each with a common "identity" running Pro.  If I look at "My devices" under "Identity" on each of the 3 computers, I see the other 2 listed, as I would expect.

The sync configuration for any given synced folder looks like this:

A <-> B

A <-> C

In other words, a folder on A has 2 peers (B and C) while that folder on B or C has 1 peer (A).

How do I achieve this:

A <-> B <-> C

In other words, how can I get B and C to treat each other as peers for a folder they each already sync with A?  The result would be a 3-way sync with each computer showing 2 peers for a synced folder.

Seems pretty basic but I'm stumped.  Thanks.