So, on the actual 3rd month anniversary, the Frog gave me a little surprise and performed his first front-to-back roll. Entirely unintentional on his part, so it'll be interesting to see when he actually makes the connection and starts rolling with intent.
Also, as an aside, I was watching this afternoon as Hubs and the Frog played together, and there's a definite pattern to the interaction. At the start of the game, the Frog looks slightly bemused. As the pattern of repetition becomes apparent, the Frog starts hyperventilating in anticipatory excitement. Finally, he'll start laughing at each game 'climax' (be it having a raspberry blown into his hand, or his chins tickled, or whatever). After this point though, he rapidly becomes bored, and stops reacting to the stimulus altogether - so we might get 3 or 4 laughs out of him before habituation kicks in, and we need to find something more interesting for the tyrant's amusement.