It's not as simple as "3 cones = 3 primary colors." Each type of cone has a response curve and three curves overlap: http://hyperphysics.phy-astr.gsu.edu/hbase/vision/colcon.htm.... And each cone has different sensitivities (blue is much more sensitive than red and green). So perfectly monochromatic light will stimulate two and usually three cones to varying degrees. When you mix "green" and "red" to get yellow, what you're actually doing is stimulating the green cones (but also the red cones) and the red cones (but also the green cones) in relative proportions that your brain perceives as yellow. But it won't necessarily give you the exact same response of the two cones as monochromatic yellow light.