I am planning to buy 5 x t3a.xlarge reserved instances for 3 years.
After the 3rd year I am planning to buy 3 x t3a.2xlarge instances to provide reservation for the t3a.xlarge instances whose reservation is still running.
Will the t3a.2xlarge provide discount to the t3a.xlarge instances?
Does this apply to RDS for db.t3.xlarge?
A Reserved Instance for the t3a family will apply to any instance type in the t3a family.
Therefore, 3 x t3a.2xlarge would also count towards 6 x t3a.xlarge (because 2xlarge is double the size of xlarge).
However, I wouldn't suggest making plans now for 3 years time because things change quickly in technology. You will probably have more and better options in 3 years.
As an alternative to Reserved Instances, you might instead consider using a Savings Plan. It is more flexible than RIs and, depending upon which option you choose, you can even use it for different instance families.
See: