You need 3 audio pairs in 1 audio pair out.
You buy thing that has 4 audio pairs + 4 Composites + 4 s-videos in and 2 audio pairs + 2 Composites + 2 s-videos out.
Yep bit over kill.
I'm not saying that is bad choice just that it's bit over kill.

You might find the box you need from pretty much any store that sells TVs.
Of course you can order the 3 way box from ebay and while waiiting it use the 2 A/B selectors.
You can easily replace the A/B selectors with the 3 way switch when it arrives.