Q: how to connect a 5.1 surround sound speakers to my iMac speakers have 3 single prong plugs

There is no way to create a real Dolby sound unfortunately. The mac currently only supports 2 channel outputs. There are interfaces you can buy that will separate the bass and send it to the subwoofer, and fake a 5 channel sound field, but ultimately it will only run 2 sets of stereo speakers, and one mono in the middle, so it is not true surround sound.

Here's a similar thread with more information if you are interested:

How can I connect 5.1 speakers to Macbook Pro 13 bought Summer 2011

I wish there were a way to do it, I personally am a recording musician and an audiophile, and have looked for the same thing on several occasions. But unfortunately at the time, it seems as though there is not the hardware to do it.