You can get extended security updates. They cost money, but there's ways around that, and Microsoft probably won't audit your 3 PCs. For a business though, yes you'd better pay or upgrade.
I checked out what one well-known torrent tracker has to say about these extended updates, and sure enough, I found a torrent called "Windows 7 Ult + KB3125574 by killer110289 (х64) (Ru) [10/04/2020] + авторский загрузчик + Office 2019 (2020.03) RePack by KpoJIuK + ESU Update март 2022" and several others which apparently have those updates applied.
I also wonder what would happen if you build a windows image out of 7's shell/explorer but 10's everything else.
I checked out what one well-known torrent tracker has to say about these extended updates, and sure enough, I found a torrent called "Windows 7 Ult + KB3125574 by killer110289 (х64) (Ru) [10/04/2020] + авторский загрузчик + Office 2019 (2020.03) RePack by KpoJIuK + ESU Update март 2022" and several others which apparently have those updates applied.
I also wonder what would happen if you build a windows image out of 7's shell/explorer but 10's everything else.