Number of RFC destinations required to be created at ECC and in NWA for sending IDOCs from ECC to PI in SAP PI Single Stack

Hi Experts,

I have read many blogs regrading IDOC configurations that needs to be done for SAP PI 7.31 single stack.

Some of them have mentioned as one, some as 3.

Even i have used 3 rfc destionations in my project.

why do we create 2 rfc destinations with same PID and one rfc destionation with out PID.

Please throw some light on this

Thanks,

Rohini