Have you copied the database frm files rather than doing a data dump? This won't work. You need to do a database dump and restore.
On the original server either use phpmyadmin or mysqldump. Phpmyadmin should be already installed by xammp
If the server is your local machine, you can access phpmyadmin via
http://localhost/phpmyadmin
Then go to database name, click the export tab and click go. This will download an sql file.
Then on the destination server, do the reverse. Go to phpmyadmim, you will need to create a blank database, then click the import tab and select the sql file.
Also copy the moodle data folder, this is a straight forward copy.
http://docs.moodle.org/26/en/Moodle_migration
Don't try to merge 3 databases, the id's will be out of sync and the data will be completely useless.
If the code from the other developers has been written correctly, they will have install.xml files and possibly upgrade.php or install.php files to make any structure changes to the database.