What will happen in this case for the RI pricing?
Only the account that has purchased the RI will get the advantage of RI pricing
One instance of a small size and running in the US-East-1a zone of each AWS account will get the benefit of RI pricing
Any single instance from all the three accounts can get the benefit of AWS RI pricing if they are running in the same zone and are of the same size
If there are more than one instances of a small size running across multiple accounts in the same zone no one will get the benefit of RI
Explanations:
Only the account that purchased the Reserved Instance (RI) will get the benefit of the discounted pricing. However, since consolidated billing aggregates all usage across accounts, it doesn’t restrict the benefit to just the purchasing account.
The benefit of the Reserved Instance is applied across the entire organization, but it doesn’t automatically apply to one instance per account; it is applied based on the instance usage and availability.
When accounts are consolidated under one billing arrangement, the benefit of a Reserved Instance can be shared across any instance in the same size and region, regardless of which account is running the instance.
The Reserved Instance benefit will apply to eligible instances across all accounts in the same region and size, even if there are multiple instances. If they match the RI’s parameters, they can receive the discounted rate.
Based on what I know, the answer is:
Any single instance from all the three accounts can get the benefit of AWS RI pricing if they are running in the same zone and are of the same size