Take a look at Red Gate tools (www.red-gate.com)
Hello Sriram
If you have VS 2010 installed you can use the data compare feature of VS 2010 to do the comparision between two databases.
data comparision in VS 2010: Open VS 2010, go into DATA > DATA COMPARE > NEW DATA COMPARISION
The approach I will follow is, I will compare two databases first and merge them into one of them (let us say comparing A & B then merge the changes into A). Now compare the other two and merge the changes into prior one ( compare A & C then merge the changes into A).
I hope it helps you out.
Are you looking to compare schema or data? if data, the following tool is much more flexible than red-gate's:
Among other tools specified in this thread, I suggest trying SQL Server database and data comparison and synchronization tools like ApexSQL Diff or Data Diff.
You can also try similar tools from Devart and see what works for you