I bought my son a gaming headset to use with his Nintendo Switch, which worked fine. However the cable wasn't long enough, so I bought an extension cable - making sure it was one with the three black lines on the connector so it would cover the L+R+mic channels - and although it still works in terms of audio and mic, he now gets a terrible ground hum effect when using it. Going back to just the headset, without the extension cable, and the hum goes away.
Interestingly, if I test the headset + extension cable on a different device instead of the Switch, e.g. my phone, I don't get the hum issue at all. There are definitely lots of posts online about people having this issue with the Switch, but they're usually streamers who wanted to split their audio so they could hear the audio and send it to their Twitch (or whatever) stream at the same time.
It seems like the usual answer to this is to get a ground loop isolator. I was looking for these, but every one that I found only seems to support two channels (two black lines), so I would assume they would not be able to transmit the microphone channel.
Does anyone have any suggestions? Are there any ground loop isolators for three channels? Any completely different approaches?