To request specific ringing pattern an Alert-Info header in the inbound INVITE to the SPA device needs to be inserted. It is specifying name of the ring pattern to play by called phone.
It is common way to implement Distinctive Ring feature. I have no personal experience with 3CX but the following may help you:
http://www.3cx.com/blog/voip-howto/caller-id-processing-distinctive-ringing-alert-info/
(search for Routing of Calls to INTERNAL Destinations WITH Distinctive Ringing)
Or ask 3CX for support.
