You could always add to the lookAt another angle to change the orientation as you desire such as:
lookAt(A.position,C.position) + [ANGx,ANGy,ANGz]
Where ANGx, ANGy, ANGz are the angles you want to rotate the layer in x,y,z axes respectively, in addition to the exact orientation the lookAt gave you.
I don't know how you want to change the orientation later, but you could use a Slider Control with keyframes to add different values at different times, and this way you could orient the B layer towards A&C at the begining and then change the oreintation as you want using the keyframes at the slider.
(If you need to change all 3 angles (ANGx, ANGy, ANGz), you will need to use 3 Sliders, or use a 3D array expression)
I hope this helped.