to covert .net project from as a sql server back end to oracle back end . what r things i need to consider for 3 tier architecture?wht about oracle connection pooling in oracle  we need almost 3000 connections for our database.........plz reply to my question as early as possible......  

ADO.NET is really good at managing the providers, so SQL or ORACLE won't make a difference on the design, if you designed the app to be working in SQL, by changing the provider won't change how to design. I would recommend to use the using{} to clean up your connections and make sure they are all back to the pools after using them