Thanks for posting.
If you have FileMaker Pro 11, you can use the Save as Snapshot Link feature to create a file that users can click on their Desktop to automatically place them in a FileMaker hosted file.
If you have a previous version, you can turn an empty FileMaker database file into an "opener file." That is, create a script using the Open File script step and then attach the script to the File > File Options > When opening this file > Perform Script checkbox. Doing this will launch the Open File script when the database is opened and will place the users in the desired database.
Concerning your second question: are the three files related to one another and you do not want the different groups to see the other groups' records? Or are they three single files and you do not want the users to see the additional databases in Open Remote?
Hope this helps get you started,