I don't know about cables. From what I have heard and from experience, gains are hard to come by when you use machines. For tri's I do:
Decline EZ Bar Tricep Extension
Exercise Data
Main Muscle Worked: Triceps
Other Muscles Worked: None
Equipment: Barbell
Mechanics Type: Isolation
Tips: Position yourself face up on an incline bench. Using an EZ bar, extend your arms until they are perpendicular to your torso. From this position, relax your triceps until your biceps make contact with your forearms. Reverse this action to return to the starting position, keeping your elbows stabilized (motionless) throughout the exercise.
source:
http://www.bodybuilding.com/fun/exer...Muscle=Triceps