How do you use Family Sharing to effectively merge multiple Apple IDs?
.The answer turned out to be only slightly convoluted.


Set up a new user on my mac, tempaccount.
Login as new user.
Use 3rd id as my icloud login.
Set up my gmail account as the Mail account.  (Since I don't intend to send email with this account on a regular basis, I don't really care if Mail.app gets confused.
Find the email forwarded from 3rd id to my gmail account.
Click and accept.


With luck this may help other people in similar circumstances.