If they are mezzanine cards they plug into a special slot/connector on the motherboard directly.
If they are vendor-specific (like Dell) then you probably will only get them to work in a dell server. If they are OCP compliant cards then you might use them in any open compute platform board. There may be able to find a PCIe to OCP card, I coulnd't find one with quick google search. For a connect-x3 it is probably cheaper to buy a different card than the adapter if you do find one.