Some excercises are supposed to stimulate the 3 heads to differing degrees, but I think its down to genetics at the end of the day. I have a small short head too & no specific training has ever changed that. Its just how your arm is put together matey.
Just stick to growing the whole tricep with some good heavy bilateral movements (dips, skullcrushers, close grip bench presses), although I have to dissagree with Vinylgroover and say that I've found rep ranges of 9-12 better for triceps growth, but this difference is probably down to individual differences in muscle composition (if it works for you, do it)