Reg, G4FGQ wrote:
"Can anyone provide a circuit diagram for a 3-phase SWR / Forward &
Reflected Power meter?"

Sure. It is provided by Bird Electronic Corporation.

Connect the 3-phase source and load in the Y-configuration. This permits
(3) 50-ohm coax cables to be used between source and load, the shields
being the common connections for the Y-configuration. Circuit delay in
the three cables must be made the same to preserve the 120-degree phase
relationship between phases. Equal length cables or phasing networks can
be used to acomplish this.

Because of the foregoing, W1, W2, and W3, measured in the loads by the
Bird wattmeter are valid regardless of power factor, degree of balance,
etc.

The total power:

PT = W1 + W2 + W3

The above is from "Industrial Electricity (Part 2)"
by Chester L. Dawes, McGraw-Hill 1942, page 141, which was one of my
first text books, and surprisingly to me, I still have it. A GI-Bill
gift from the U.S. Government at the end of WW-2.

Best regards, Richard Harrison, KB5WZI