Welcome, @VNS, to the OEM forum.
In the absence of any hint as to what your supply is, I’m assuming it is the standard UK 240/415 V, 50 Hz. I’m also assuming your use and generation are split roughly equally across the three phases.
Two choices spring to mind, our home-grown emonTx or a commercial meter.
For the emonTx, it looks like 3-off, one per phase. Each has one voltage and four current inputs, of which you’ll use two. The voltage input accepts a nominal 9 V from an a.c. adapter to measure the line - neutral voltage, the current inputs are scaled to accept a 100 A c.t; while that is plenty for a domestic consumer, I’m guessing that your peak load is a lot more than that - so I quickly add that provided you have the expertise in-house to (possibly) change the burden resistor inside the emonTx, you can use almost any c.t, though if you can stay with a 50 mA secondary, no burden change is necessary.
I can only guess which c.t. you might need for the “consumption” (though whether that’s consumption or “nett grid” depends on where the PCC for the PV infeed is and where you can put c.t’s), but it’s likely to be similar to the ones for the PV infeed. At a possible 280 A or so, it looks as if the PV infeed is going to need a 300 A c.t. per phase. We list a few suitable ones in the Learn section on the “Use in N.America” page, a likely candidate is the YHDC SCT023R - the 400 A version (the 200 A rated one has a max current of 300 A so unless you’re on peak PV output, it is probably OK, but you’d need to change the burden because the input would be clipping and giving bad readings). For what you’d gain in sensitivity/accuracy at low currents, it’s probably not worth it.
The emonTx by default sends the data by radio on the 433 MHz ISM band to an emonBase - which is a Raspberry Pi fitted with a receiver. That’s where the data is stored and made available via web pages.
If the ISM band radio doesn’t suit, the preferred alternative is a short (a few cm) serial link to a Raspberry Pi Zero connected to your LAN, then there’s no need for the radio on the RPi that’s the emonBase (and then you don’t need that either - it’s possible to run emonCMS on any server - but running emonCMS on the RPi is a case of insert the SD Card and power it up, on anything else can be trickier. Either way, you still need to set up the maths you want to do on the data coming in, what you want to store and what you want to display.
An alternative to the emonTx would be stand-alone secondary meters (DIN-rail?) connected by a data bus (Modbus over RS485 is a common one). You’d need one 3-phase for the PV infeed and a second for either Nett Grid power or consumption (as with the emonTx, that depends on where you can put the c.t’s). And then it’s a matter of a RS485 - Ethernet converter to get the data into your RPi. The software to do that is relatively new, I’m not sure where it’s at in terms of development/experience in use.
Secondary meters offer guaranteed accuracy out of the box, whereas the emonTx will need calibration to achieve the same accuracy.
I’m not sure what you mean by secondary sites? If these are near copies of what we’re discussing, then I guess that a VPN to allow remote access to the remote server running a separate emonCMS is what you’re looking for?
I can’t really answer “How to set it up?” because I don’t know the details of what you want. Probably a little time looking at the Guide and maybe, if you can find discussions on the forum where there’s something similar, that might help.
As always, come back if you want any more help. I’d much rather help now than find you’ve bought the wrong things and have to tell you that later.