Is it just a single 3.5mm jack to connect them? I've just done a quick Google search and read that it's a single cable connection to your PC, but it didn't say what.
It sounds to me like you need a digital output, so if your motherboard doesn't have a digital output (like most modern Creative cards do; SB Live! 5.1 onwards certainly) you will need a different sound card to use those speakers.
That's assuming that you are talking about the onboard sound on an Asus motherboard. If there are only three jacks they will probably be Line In, Line Out/speakers and Microphone or something along those lines.
Do you know the model number of your motherboard?