Hesitating in case 3 is clearly unethical, as West has no problem in playing from either T9 or 67 in this scenario. One may hesitate in order to determine which card to play from a technical perspective, but not whether or not to signal to partner without an accompanying technical problem. Otherwise one would always hesitate when showing an even number with 2, and never hesitate when showing an even number with 4, thus creating a(n illegal) signal that clearly distinguished between holdings of 2 and 4 cards based on the manner in which the cards were played rather than the particular cards played.
Even in the absence of the above, hesitating would clearly be seen as an attempt to mislead declarer on the location of a single card between the A and 8. It is quite likely that Declarer is not harmed in this instance, but that does not preclude the award of an adjusted score against the offending side, EW here. The appropriate penalty for a first offense would be the actual score for NS, and lower of actual and Average-minus for EW.
Update - Added comments from below
1) "False carding to make the declarer lose count" is not illegal in this instance, but "reinforcing" it by hesitating is, right? – Tom Au
Yes; but it is important to realize that all signals not made in tempo comprise Unauthorized Information (UI) for partner; namely that there was a question about whether or not to make the signal, and thus that the signal is in some fashion weak. Likewise false carding must be done in tempo in order to not be seen as an attempt to improperly deceive declarer; but this is really just a special case of the preceding as partner is also given UI.
2) In example 2 (and even example 1), you have two cards that "straddle" the 8, so you have a technical problem, right? Then hesitating under those circumstances would be amateurish, but not wrongful, right? -- Tom Au
Yes, that is correct; however the decision on carding if Declarer plays low towards Dummy should have been made as part of West's review of dummy right after it came down. Hesitating when the play occurs is still UI for partner, and will hinder effective defense in consequence. Practice taking a full 3/4 second or so on every play as defender, to give yourself a couple tenths reaction/thinking time when you need it.
Yes, the experts do what you suggested, and it's the amateurs that wait until it's too late = Tom Au