Am I correct in assuming you need to order the KXV3 transverter interface in order to get the RX ANT input?
Please refer to the antenna switching diagram for possible antenna combinations. You can connect a dummy load to the main antenna jack (for safe TX tests at high power), but connect a real antenna to RX ANT IN, and simulate how full QSK will sound without actually putting out an on-the-air signal. If you connect an antenna to the main antenna jack, and connect RX ANT OUT to a second receiver/transceiver, you can A/B test the two rigs just by tapping RX ANT on the front panel. With RX ANT OFF (normal), the K3 will get the antenna; with RX ANT ON, the other rig will get the antenna.