I tried to multiply the AO on top of the fill lights and then add the key lights on top of that. I'm posting all the render layers so you can see whether or not I did it right. I am confused about whether I should be using render passes - right now I am only using render layers. Could you go over render passes briefly? Thanks!
Final Composite
AO
beauty pass without AO
fill lights
background (something went wrong here - it's not as blue as in the beauty pass)
key lights
