I would say three is already overkill but the max that can be justified. With the proper anti-emetics two glasses should be the max. I suppose without good anti-emetics a third glass could be justified.
Not really. Like I said, it's very rare but some people might throw up a third time. Typically at this point though the body has absorbed enough from the first 2 that a 3rd shouldn't be needed hence the "better safe then sorry".
Also even without the other medication 1 drink is still quite possible. The meds really aren't much more then comfort (beta blocker for heart rate example.) and I'm fairly confident that while yes it probably happens less often, there's still a chance of throwing up.
Personally if I could have gotten it I wasn't going to bother with the meds. (aside from what I might have on hand. Like I use to have olanzpine that would have me sleeping all day, that sounds useful.)