I had a 3 meter config last year on my old home. One of them was the regular utility meter on which I am billed, another measured solar generation only, the third measured the power going in and out of the power wall. This configuration was required by my utility in the old Powerwall 2 implementation design wherein all solar power did not flow through the Powerwall before powering the house or going to the utility grid.
This year, the implementation at my new home has only a two meter config - one of them the regular utility meter that is used for my billing, and the other meter measures the solar generation cum battery charging. With the new design of Tesla where all solar generation flows through the Powerwall+, Tesla told me that only one second meter is sufficient, and a third meter is not needed. I am surprised that you have a Powerwall+ and they need a third meter.
Right now I only have 2. The utility one that needs to be replaced by the net meter and one right below it. I am not sure it serves any function except to allow them to put the unit that cuts power to the grid when the grid is down “under the meter”. Because of limited space they had to locate the shutdown and rapid shutdown and placeholder box with a sticker reading “photovoltaic system dedicated KWM METER” on it about 6 feet away and the powerwall+ a further 6 feet away. They said they will install that missing meter once they get it… but that the utility should still replace the meter and give PTO without it… I messaged Tesla and they just said to wait to see what the utility do…