I use an older Heil pro set with HC-5 element on my K3, with great results.
One nice thing on the K3 or K3S is that you can plug the two 3.5mm plugs
directly into the back of the rig- no adapters needed, and no cables
hanging out the front of the radio.
However, my advice to anyone is that the most important thing to look for
in a headset is COMFORT. The audio coming and going can be tailored to your
liking for nearly any mic and headset using the equalizer in the K3, but if
you can't comfortably wear it, even the most expensive headset is useless.
I use the Yamaha CM500 from Amazon about $58 with my K3. I bought the Koss SB45 from Amazon about $23 shipped. I think the Koss is every bit as good as the Yamaha. I haven't used the Koss on the K3 but expect it would perform equally. I use it on my Icom 7300 and get nothing but good reports. I get as good a reports with the Yamaha as I did with my Heil of which I've had a couple.