Ours was an audio cable with standard phono jacks on each end. It was a mono version not stereo. Not got a pic as the cable is on the boat. I think I bought it from Maplins for a couple of quid.
I had a mono 3.5mm jack to 3.5mm jack connecting the hf3 to a laptop. I soldered a 1:1 isolating transformer in the middle to stop any noise from the laptop effecting reception.