Close grip is king for lateral development. Make sure you're not allowing the elbows to flare out, but keep them close to your lats during the movement. Grip no closer than the width of your lats.
For the medial head, you'll want to relax the long and lateral head, so pushdowns or kickbacks (elbows close to or even behind your torso will be most efficient).
For the long head you'll want to prestretch it. Overhead extensions will do the trick.