One of the nice things about the elecraft K3 is the support that it has for a very wide range of microphones. The rear panel mic connector is directly compatible with computer headsets once the bias voltage is enabled to power the electret condenser element.
My normal headset is a home brew one based on the bias circuit that is normally used in Icom hand mics. This simple circuit is published in the user manuals for most modern Icom Radios. The manuals can be found on line on the Icom website.
Recently, however I have been using a little headset that I bought on Ebay for 1 pound Sterling ( about 2 US dollars.)
I have been getting truly fantastic audio reports on this headset when used with the K3. It’s a little bit flimsy but it’s very light and easy to wear, It simply loops over one ear.
Vox on the K3 is fantastic but I prefer to use a footswich for PTT control.
Lots of Ebay vendors have similar products and any computer store will sell you a ‘skype’ headset. Before you spend ‘proper’ money on expensive ham radio branded products, try a cheap computer headset. You may be very pleased with the results.