It's a fresh install with the latest drivers and Boinc and updates.

Which tells people trying to help exactly nothing.
Is it an Nvidia GPU, or AMD, or Intel? Driver version?

The AMD drivers are very unstable since they changed over to the newer Adrenalin 2020 version. I have crashes in the driver and videocard when it's just sitting on my desktop and I am not using the system. Type reliability into Windows Search to bring up your reliability history chart to see what is all crashing. It may show hardware errors, but those can just be drivers crashing, nothing to do with hardware.

It can also be that your Windows isn't so fresh anymore. In that case see https://www.windowscentral.com/how-use-dism-command-line-utility-repair-windows-10-image and follow the Repairing issues with DISM using RestoreHealth option and How to run SFC to repair problems on Windows 10 options to fix possible problems with your WIndows 10. One of my systems which had a new Windows 10 Pro installation on it got fixed that way. Just telling that even completely new Win10 installations don't mean your system is stable.

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How can it not be fresh when I only installed it a week ago? I've run DISM and SFC anyway, and they both found no problems.

Depends how old your installation medium is. Even if you downloaded it the same day, it might have been sitting on the server for weeks, months, years. Did you keep forcing Windows updates until it said there was nothing left to do?

I'm using rubbish power supplies at the moment

That can cause wild stability problems with 3 GPUs in the system of course. That new PSU can help a lot.

What is the exact Windows version and build over there? Press WIN key and type cmd, then hit ENTER and type winver and hit ENTER.
If the version on the DVD is very old it has lost its ability to update itself by now. It would remain outdated even if you kept clicking the 'update'.

Version 1909, Build 18363.657

Yes that looks good! Update did work for sure. There are still Win 10 dvd's circling around with old version 1511 or 1703 in them. A new installation from that kind of media wouldn't get update for the updater itself nor major upgrades to never versions. They might get some old updates but never be "up-to-date" anymore.

That seems strange that older ones won't update. Why on earth would Microsoft do that?!?

I've understood that the update infrastructure which Microsoft is running these days and the internal architecture of Win 10 have changed so much in 4-5 years that those oldest versions would require a massive amount of patching and stiching before those systems would even understand what they are connected to. The update process has been redesigned completely. Then... updating those old junkyards (1511 at least) would basically be the same as a complete wipe and installation of the OS. I think it's wise for Microsoft if they don't support online upgrading straight from those old versions anymore. That saves their resources.

I'm not sure if those systems can be upgraded by using the MS 'Media Creation Tool'. I'm pretty sure that "upgrade" would then again be a complete wipe and installation of the OS. Not different than a fresh installation to a new drive. I would't trust for a second that my own files would be found anywhere on the connected drives after that process, even if the process gave an option to keep my files.

At least the Media Creation Tool fits on a 4GB USB thumb drive. All you need is a USB 2.0 slot, or faster (preferably faster). And the one you download is always the latest. (well, bar the updates that have come out since)

You asked how can a fresh install of Windows 10 be corrupt? I wonder that myself, but it happened to me on one of my machines. That also had a one week version of Win10 Pro on it, but DISM and SFC /scannow found faults to be fixed. And since that fix that system is stable, where it wasn't before.

And I hate trying to make USB sticks bootable :-/ It never seems to work, either I do it wrong or the BIOS don't like booting from there. I like to start from a DVD.

use a tool called Rufus. makes it really easy on Windows. just give Rufus the .iso file and tell it what USB drive you want to be bootable with that .iso and it does the rest.

I like to start from a DVD.

Trouble is, at least with desktop PCs that you're hard pressed at finding a DVD or Blu-Ray player on board these days, what with all the large RGB fans and radiators at the front. There's no room anymore in most cases to put anything protruding from the front, so then you end up attaching an external player via USB, since these cases have an abundance of those ports on board. And then it's just as easy to use a thumb drive instead of a (scratched!) DVD.

(My Fractal Design case has 2x USB-A 2.0, 2x USB-A 3.2 Gen 1 (5 Gbps), 1x USB-C 3.2 Gen 2 (10 Gbps) on the front and the motherboard has a further 1x USB 3.1 Gen2 Type-A port (10 Gb/s), 1x USB 3.1 Gen2 Type-C Port (10 Gb/s) and 6 USB 3.1 Gen1 at the back. It's USB heaven!)

Seems like you found your error. I've noticed multiple times, how PSUs can be unstable when running them near the max of their rating.
An 800W PSU, you best run at 300-600W at the wall (400-500W being optimal).

I would second, a USB Blu-ray RW, since they're only a few bucks more than a DVD player, and can do Blu-ray, and read and write (if they can do M-Disc is even better).

Seems like you found your error. I've noticed multiple times, how PSUs can be unstable when running them near the max of their rating.
An 800W PSU, you best run at 300-600W at the wall (400-500W being optimal).

I would second, a USB Blu-ray RW, since they're only a few bucks more than a DVD player, and can do Blu-ray, and read and write (if they can do M-Disc is even better).

The new PSU has helped dramatically, Corsair 850W, actually running at 870W at the wall until I get the LED 1kW supply to assist. But Corsair are made properly, if it's too much for it it will shut down instead of blowing up. But 12V, 5V, 3.3V are all correct unlike the cheap supplies. One day I'll have a check to see if the Corsair supplies a proper 12V with no 5V load. The CIT supplies did not.

But it still crashes occasionally. I've now moved one of the three GPUs to another machine (but still off the same PSU), to find out if it's the GPU, the motherboard, the PCI Express adaptor, etc. So far it's running for much longer so I'm nearly there.

I hardly ever use DVDs, only on the odd occasion I want to run Memtest or install an OS. I've got two SATA DVD recorders, that will do me. And since SATA is hot pluggable, I can even remove it from a machine while it's running.

I would advise against running the PSU for prolonged time at those wattage ratings. It'll shorten the life of your PSU and possibly hardware as well.
Plus, there's a chance PCIE risers will act unstable when all hardware is maxed out.
Each GPU can easily be trimmed down by 10-25% of power consumption, without any performance loss. (up to 60% on some projects with a RTX 2080 Ti).
if you're planning on running those wattages, it's best to run a 1000-1500W PSU.
I run dual 1000W PSUs in mine (one per motherboard), and limit my GPUs to 4 per board, but run them usually around 800-900W max at the wall (because my UPS is rated up to 900W (but can still handle 975W)).