which hardware is good for linux pc server?

Any old computer is good for a server in the case of a cardserver.

No need to use 3 computers, one is fine, it's all down to how much you read and getting your "own" setup correct.

Any computer which could run a windows operating system will suit your purpose, so long as it is set correctly and it has the correct drivers to get everything working. If it is a old PC or not a very fast one, then it is an idea trim down what you are installing on it, as to just run a server, you don't need whistles and bells.

It might be an idea to look at the system BIOS to ensure everything is how it should be and there is nothing going on which would cause things to behave differently, not sure if there would be any USB or power saving options in the your BIOS but worth a look. Usually if you are having issues, then it would be software driving your hardware rather than hardware itself. If you are getting different results on different computers, then you might want to see if you need additional drivers for your chipset, so might be an idea to browse the manafacturer websites to see if there are any additional drivers. Also maybe a browse of linux forums to see if anyone else has any issues with USB devices on your computers. If any drivers are needed that are not under the GPL (open source) then they would not be installed by default. To be honest though with USB, they are all set to a standard in such a way that they should all work without anything additional, after all, it is a universal standard format designed so that lots of things work, so maybe there could be some other issues causing it rather than the USB itself.

Worth looking at any power saving options you have set up, I have found that any issues with my USB devices is often down to this.

Hard to tell what is going on but I am sure something will be able to fix it!