For it to affect two of the three computers but not the third would suggest it's down to something installed on those computers but not the third.
When the connection goes down, have you tried the troubleshooter to see what that reports - Event Viewer may also have something.
Generally with Internet problems, I would start with the drivers by going to Device Manager/View/Show hidden devices/Network adapters and right click on whichever adapter you are using and select Uninstall, but
do not check the box to include software.
Either a reboot or clicking on Action/Scan for hardware changes will reinstate the driver.
When it does go down, it may be worth going into Device Manager to that extent anyway to see if there are any yellow alerts.
To determine if it is something installed on those computers, trial run both of them in Safe Mode with Networking.
If it doesn't occur in that mode then you will have to troubleshoot with a clean boot.
While overheating of a router can cause connectivity problems, I'm not sure if overheating of a computer would have the same effect, although I've read of it as a suggestion.
Can you check the temps on both of those machines, although a telltale for that is prolonged increased fan speed.
I use the free version of HDMonitor to check that sort of thing on my laptops.
The free download button is down on the left -
https://www.cpuid.com/softwares/hwmonitor.html
