Hello,
i have a ring group with 3 extension (1001, 1002 and 1003). I need a hint to setup something like a “on hold” so if all the extensions are busy, the caller will hear a song, or a message, and when an extension became available, the incoming call will be redirected to that extensions.
I`m quite new in freepbx, so please be gentle 
I hope you mean gentle - my religious affiliations should have nothing to do with my choice of FreePBX… 
Set up a queue instead of a ring group. Much more capable, more options, music on hold, you can enable position announcements, and you can give the customer an option to dump to voice mail.