Can I output to a 3.0 speaker setup from an iMac?

I play movies stored on my iMac through my speaker system and HDTV using a Mini Display to HDMI cable plugged into my reciever. I'm currently piecing together a 5.1 seperates system but currently only have the front left, front right and centre speakers (3.0). I've played around to try and get the iMac to output though all three speakers but the audio only comes through the left and right channels and ignores the centre speaker completely. Even when I set the output in the 'Audio MIDI Setup' to 5.1 it still doesn't recognise a centre channel.

Any suggestions? Stereo output is fine but it would be nice to get the centre speaker in use rather than just sitting there in silence.