Stereo sound separation into three sources
Partially correct. Human auditory spatial perception is rather complicated. A good book on the topic would be Spatial Hearing.  How and where we localize things depends on a great many factors. For left/right localization the main effects are interaural time difference and interaural level difference. Time differences is the strongest queue. Whatever comes earlier on the left ear will be solidly localized on the left unless even if the sound is louder on the right (within reason). 
If left &amp; right signals are perfectly time aligned than your model is simplified model.