Due to challenges in obtaining the 3.5mm (1/8 inch) jacks, Midway had to purchase only what was available at a reasonable price. There are three types of 3 circuit jacks we ship with our kits. The kit uses two jacks, one for the headphones and one for the key. We try to make them both the same when we kit the units for shipping.
The headphone jack is a stereo type so you can use typical MP3 type stereo earbuds or headphones. In this case you wire the left and right channels together. These are typically called "tip" and "ring" for left and right respectively. The only caveat is that if you use a mono plug when wired this way, it will short the audio output to ground and you will hear nothing. It won't damage the output circuit because of the protection resistor on the output opamp.
The key jack only needs one line to ground so you wire the "tip" and "sleeve", the tip being the keying line.
The three possible types of jacks in your kit are pictured below.