Would a CAT 6 Ethernet cable split into 3 devices (two consoles and a pc) work with no issues at all? If so how much would ping/download speed be affected? I want to run a cable from upstairs to downstairs and I need to make sure I could theoretically use all 3 devices at once on one cable (watching streams and playing online, etc). I currently have a 200mbps download speed. Cheers.
Don't even think about physically splitting an ethernet cable. They are not designed to be split. To achieve what you want run one ethernet cable to the new location and then install an unmanaged ethernet switch. They are not expensive, around £10-15 and designed to do precisely what you are wanting. Splitting a single ethernet cable would cost more take much longer and not be reliable.