Updating a unconnected record using pipes

Three tables
A is parent
B is the child of A (one to many)
C is the child of A (one to many)
Open a details page of A and pass the record ID of B in the URL.
On this details page, I am inserting a new record in table C. For workflow application reasons, B is connected to both A and C.

From another post, using the URL value to update a join isn’t possible (please let me know if I am wrong) so I have a placeholder field for the record ID of B in the C Record that is populated with a script.

Is it possible with pipes to grab this text value to update the B record and skip trying to join the tables for a one time update (literally the only time the tables need to interact)?

Another option would be to update the join field in C to B’s Record value with a script so any ideas there would work as an alternative.

It’s clear as Mud to me…