Take each of the microphones and connect them to an input on your mixing board. While mono by nature, each of the mics connected will allow you to pan them as you want, creating a fuller spectrum with stereo sound output.
Adjust the levels of your mics, both volume and pan controls, to the sound you desire. Also adjust the main output volume on the mixing board to avoid too low of a signal or distortion in your recordings. Plug your cable into the RCA output on the mixing board.
Plug the other end of your cable into the PC mini audio jack on your computer. Open your recording software, and select the correct input from the "Sources" list.
Speak into the microphones to observe whether the levels on your software are accurate, and do a test recording, speaking into and tapping each mic individually to ensure each one is being recorded.