Add a third speaker to audio interface

[Reggiejackson]

I'm using a pci audio interface [[ Marian Seraph AD2] that has two cable for 2 Analog In- and Outputs (4 Channels)and a set for 1 AES/EBU In- and Output (4 channels)
that come out of it . I'm only using the analong outputs I don't know how to use the aes. I use it for instruments and to play music. I would like to add a third speaker and have the third speaker only play my amp sims while the other two play my amp sims and a music player as well. I got no idea how to do this though.. I can add a sub woofer pretty easy but not sure how would work with a regular speaker..

Thanks for any help.