Yep, you can get a phono switch box.
Something like this...
http://www.maplin.co.uk/Module.aspx?ModuleNo=23189
So the left & right phonos from the BT Vision box, and the left & right phonos from the PS3's AV lead, would connect to the phono switch box.
The switch box's output phonos would then connect to the 3.5mm adaptor, which would then plug into the "DVI Audio In" on the back of the TV.