I think you're making an incorrect assumption about which is the most plausible method for calculating the hourly retrieval rate.
The most obvious way to me would be to assume it is based on the actual amount of data transferred in an hour less the free allowance they give you. Which is actually what they say:
"we determine the hour during those days in which you retrieved the most amount of data for the month."
This also ties in with what the cost is to them, the amount of bandwidth you're using.
In your example you would need to be getting transfer rates of 3TB/hr. Given the nature of the service I don't think they are offering that amount of bandwidth to begin with. (I'm sure they get good transfer rates to other amazon cloud services but customers could be downloading that data to a home PC at which point they will not be getting anything even close to those transfer rates)
At that point a bigger issue might be how long it takes to get the data out rather than the cost.
At an overly generous download speed (residential cable) of 10GB/hr your 3TB archive would take over 12 days to download.
Given tc's edits above regarding additional charges for transferring out of AWS I'm starting to change my mind. I still can't believe amazon would ever end up charging north of $20k for a 3TB retrieval but it seems the intended use-case (as enforced by pricing) would be write-once read-never! Other use-cases are possible but as others have noted you would want to be very careful how you go about setting it up to avoid getting some ugly charges.
The most obvious way to me would be to assume it is based on the actual amount of data transferred in an hour less the free allowance they give you. Which is actually what they say:
"we determine the hour during those days in which you retrieved the most amount of data for the month."
This also ties in with what the cost is to them, the amount of bandwidth you're using.
In your example you would need to be getting transfer rates of 3TB/hr. Given the nature of the service I don't think they are offering that amount of bandwidth to begin with. (I'm sure they get good transfer rates to other amazon cloud services but customers could be downloading that data to a home PC at which point they will not be getting anything even close to those transfer rates)
At that point a bigger issue might be how long it takes to get the data out rather than the cost.
At an overly generous download speed (residential cable) of 10GB/hr your 3TB archive would take over 12 days to download.