database design from existing DBases

Hi All,
There are 3 applications each one of has respective databases.Now i am trying to design a database which should capture the important entities from these systems.
what is the best approach to follow?
Thanks,
rajHave you defined the important entities that need to be included in your new database?|||I am in the process of identifying the entites.
The problem is : 3 legacy applications store the same piece of information with different names in their databases.

I am building a new application taking attributes, union of all systems.
because there is no DB design design avaialble for existing systems.
I am using profiler and tracking the queries fired on to server and identifying the attributes and entities.

Is there any other best approach for doing this?

Thanks,