Hi,
Most of the information for 3PCC is available in the doc of which you may have already references above from chapter 5:
www.dialogic.com/.../globalcall_for_ip_hmp_v12.pdf
You can certainly use GC API still in this case to handle the call control, but the main difference here is that you have to control the RTP stream as well directly via IPML API for starting and stopping that stream. In additional to complete access the SDP sent in each SIP message of the exchange.
Also there is a example of handling SDP API demo in the following location of the install:
C:\ProgramData\Dialogic\HMP\demos\sdpapi
There is a project which can be used to build the SDP api lib and then that gets used by the demo (subfolder) provided which runs in 3PCC mode. Also there is a different doc folder that has information pertaining to the complete project.
Jeff