What does 3PCC mean?

All call handling actions (answer, hold, initiate transfer, etc.) are performed on the SIP endpoint

All call handling actions (answer, hold, initiate transfer, etc.) are performed on the multiple PCs (In this case 3 PCs)

T-lib signaling takes over SIP signaling, so third party application such as Agent Desktop can transfer VoIP packets to SIP Server and back using T-lib

A T-lib client such as Agent Desktop can control a call which is actually between other parties such as SIP endpoints