3-tire architecture is very simple. It is for Separating Database, Business Logic, UI. You have to write a code based on that.
1. Write all the DB Logic(Insert,Delete,Update,Get etc..) in Stored Procedure. Do not write any inline query in your business logic. In case if you want to change the db type(Sql Server to Oracle .. ect ). It will be easy. You do not need to touch your business logic and UI.
2. Keep all the business logic in the separate dll. In case if you want to change any calculations,process etc. It will be easy to deploy. Just change that dll and update. Do not need to touch the UI and DB.
3. Keep the UI as separate application. It will be easy to change the UI( Colors, Layouts, etc.. ). You may use more that one UI(Mobile, PC etc..) having same (BO and DB).
If you write the code like this, It will be easy to development also.
UI developer do not worry about the DB and business logic.
DB delveloper do not worry about the UI And Business logic
BOL developer do not worry about DB and UI.
Direction is important than speed