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: https://www.magnelab.com/products/potential-transformer-output-proportional-input-spt-0375/ 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.