T3 RDS instances are not yet available anywhere (that I know of), only T2. T2 is not available in eu-north-1.

https://aws.amazon.com/rds/postgresql/pricing/ https://aws.amazon.com/rds/mysql/pricing/

If you were running your databases on T2 instances on regions supporting it, migrating to the cheapest available RDS instance type in eu-north-1 would cost $0.374/hr on-demand. That's over 9 times more than the price of db.t2.micro in eu-west-1.