Well odds are that plug going to your speakers is a stereo plug, so it has two channels in it. What you need is an adapter that goes from 3.5mm female to two 1/4 TR males. Can the cable that goes to your speakers be unplugged so you have just a female into the speakers?
Ya, the adapters are only a few bucks, which you will have to pay for the new plug anyways, and that way you have the flexability to use it in either configuration. I personally always carry at least a half dozen adapters (1/4", 1/8", RCA, etc...) with me on any show even if i dont expect to use them b/c you never know when you will need one.
I would get a 1/8th inch stereo male to 2 male RCA. I would then use 2 female RCA to two 1/4 inch mono male connectors. If you have stereo channels open on your board this will preserve the stereo image using them, if not it will take 2 mono channels to preserve the stereo image.
If you dont have a stereo system, then only plug one side in and still have the flexiblity to use both if the need arises I have even done crazy things with running two media players on my computer and having each panned to one side, so I have control of each seperately on the mixer.
Anyways, you were talking about computer speakers, so those are most likey stereo and if you board has stereo outputs it'd be worth it to go with the stereo option so you can pan.
Usually I like to keep both channels when going into the mixer. It always sounds odd when there is supposed to be something on the left or right channel of audio only and you're missing it because you have the wrong channel. In a pinch there isn't much one can do though.