This is what I use in my .emacs to work with BBDB-3 in Gnus, it will give this functionality to the ; key:
(require 'bbdb-autoloads)
(require 'bbdb)
;; initialization
(bbdb-initialize 'gnus 'message)
(bbdb-mua-auto-update-init 'gnus 'message)
;; size of the bbdb popup
(setq bbdb-pop-up-window-size 0.15)
(setq bbdb-mua-pop-up-window-size 0.15)
;; What do we do when invoking bbdb interactively
(setq bbdb-mua-update-interactive-p '(query . create))
;; Make sure we look at every address in a message and not only the
;; first one
(setq bbdb-message-all-addresses t)
;; use ; on a message to invoke bbdb interactively
(add-hook
'gnus-summary-mode-hook
(lambda ()
(define-key gnus-summary-mode-map (kbd ";") 'bbdb-mua-edit-field)))
I got this information from somewhere on the net, but can't quite locate where just now; maybe this can set you on the correct path?