EC2 Reserved Instance Sharing across Accounts

If you have Reserved Instances and Saving Plans sharing on for all accounts, you have no control which account the RI will apply to first if the eligible EC2 instances launched on both accounts. It is recommended to purchase RIs in linked account incase that control needed. In your case, if you only have 3 accounts and there is no need for Account #2 to use the RIs, you can disable RI and SP sharing with account #2. This leaves the RI available to Account #1 and Account #3. You can do that by going to this link in your account: https://us-east-1.console.aws.amazon.com/billing/home?region=us-east-1#/preferences

If you want to verify that Account #3 is covered by reserved instances on specific EC2 usages, In Account #3 navigate to:

• Billing and Cost Management
• In the navigation pane to the left, go to Reservations, and choose Coverage Report.
• Here look at the Instance type which matches the RI and check if the coverage apply and at what percentage. You can view the same report from Account #1 by using the Linked Account filter in the right of your screen.

As long as the accounts are part of the same AWS Organisation and Reserved Instance and Savings Plans discount sharing has not been deactivated (the default is that is is active) then the discount for using reserved instances will be applied across all accounts that are part of the organisation.

Your situation is similar to this example here https://docs.aws.amazon.com/awsaccountbilling/latest/aboutv2/consolidatedbilling-other.html#consolidatedbilling-ec2

If your bill is higher than expected then you can always raise a billing support case https://support.console.aws.amazon.com/support/home#/case/create