Does bbdb3 have something similar to bbdb/mail-auto-create-p from bbdb2?
Basically I just want bbdb to be big brother and auto-create entries
for every email address I ever see. Ever.
Bonus points: do this on a per account basis (e.g. I want to add all emails ever seen on an email sent to my work address, but not necessarily my personal address).
You can do the following:
(bbdb-mua-auto-update-init 'gnus 'message)
(setq bbdb-update-records-p 'create)
For additional options, the variable bbdb-update-records-p has some nice documentation (C-h v bbdb-update-records-p).
To limit how you create records, you can look at bbdb-accept-message-alist and bbdb-ignore-message-alist. For instance, to avoid recording addresses for messages to your home address, you could try:
(setq bbdb-ignore-message-alist '((("To" "CC") . "email@home")))