This would be a very complex task to do and would require a good knowledge of SQL Server Databases.
The data from the other databases would need to be imported in to the database, but this would drop much of the database schema such as default values, primary keys, foreign keys, indexes, etc. that would need to be manually rebuilt.
I am a developer on Web Wiz Forums and even with my knowledge of the database schema this tasks to merge 3 forums in to 1 database would take the best part of a day to complete correctly.
If the forums are large, or could become large I would not suggest using a single database as this could impact on performance.