So - what is your difficulty? On page 9, there’s an application circuit diagram, showing the voltage inputs and current inputs, and optical isolation on the digital outputs.
SAFETY WARNING
The GND connection shown means that the whole circuit up to the opto-isolators on the outputs is live to Phase B. You must not test nor work on this circuit as drawn unless you are experienced and competent to work on live apparatus.
How can I do connection with 3P3W.
As there are only two wires A & B, which are connected.
How should I connect B, as those MCU pins are connected to GND.
Does that mean I don’t have to connect one of the wires?? If yes, then how do I determine which one not to connect??
Have you read the page in ‘Learn’ about 3-phase power? When measuring any power, one of the very basic facts is this: you need one less wattmeter than the number of wires. Now one channel of your M90 counts as a wattmeter, so the diagram is perfectly correct: it treats Phase B as the reference or common wire (and you don’t measure the current in that one) and it measures the currents in phases A & C, and measures the voltages between A & B and C & B. And this is why I warned you about safety:
The “GND” on that diagram is LIVE to phase B. If you do not understand that, you risk electrocuting yourself.
I am a Chartered Electrical Engineer, I don’t think you understand the danger that you are putting yourself in.
If you must continue with this project, then I think you must use an isolating transformer to measure the voltages, and not make a direct connection to the mains electricity. That will be expensive, but how do you measure the cost to your family if you lose your life?
I think you need two of these: SPT-0375 Potential Transformer Output Proportional Input Magnelab to replace the direct connections via the chains of resistors R3 - R10 and R25 - R32.
Connect the BRN wire of the first SPT-0375 to Line A.
Connect the BRN wire of the second SPT-0375 to Line C.
Connect the BLU wires of both SPT-0375 to Line B.
Connect the WHT wire of the first SPT-0375 to the VAP input (Pin 13) of the M80.
Connect the WHT wire of the second SPT-0375 to the VCP input (Pin 17) of the M80.
Connect the BLK wires of the both SPT-0375 to GND.