Actually the lateral head is hit most often, such as with chest press and tricep press downs, as well as DB extensions. Technically, though, they all fire, all the muscles fire...there is not a real differentiation between one exercise hitting the 3rd head more than the others, but you WILL feel more full recruitment if you do OVERHEAD TRICEP PRESSES/ FRENCH PRESSES/ OVERHEAD TRICEP EXTENSIONS.
The same nerves fire off to the same heads of the muscle to recruit a muscle to contraction. Varying your position cannot alter how that muscle is recruited (ie: you cannot technically use 2/3 of that muscle -- you either use it or not). It comes down to what your body has adapted to.