I got this agent 3way conference leave recoding working with :
exten => _9X.,1,AGI(
agi://127.0.0.1:4577/call_log)
exten => _9X.,2,Dial(DAHDI/g0/${EXTEN:2},,TtoR)
exten => _9X.,3,Hangup
exten => _71.,1,Set(AUDIOHOOK_INHERIT(MixMonitor)=yes)
exten => _71.,2,MixMonitor(blah.wav)
exten => _71.,3,Dial(DAHDI/g0/${EXTEN:3},,TtoR)
exten => _71.,4,Hangup
exten => _71.,n,MixMonitor(blah2.wav)
or simply call the MixMonitor in agi-NVA_recording.agi file.