1. Depends. If the 3 SQL DBs are on different machine, you can just use one config file but you change the values when you switch machines. If the 3 databases are on the same machine, you need 3 config files and you send the correct one along when you start the package. Or if you need to access the 3 databases at the same time, you can just use one config file with 3 seperate configurations (assumuing you have 3 connection managers).

2. If you move the package, the package still needs to be able to find the config file. If the config file is for example at D:\Config\Config.dtsconfig, the package at the new location will look for a config file at the same path. Relative paths are tricky, I think you need BIDSHelper to work with these.