Any time you extend your elbow joint, your triceps brachii muscle will all fire simultaneously.
However, you can alter your form in order to maximize tension on different regions of the triceps brachii (long head, medial head and lateral head). A common misconception is that doing “overhead” extension is “best” for the long head. Although overhead extensions overload the lengthened portion of the range, they do not and can not overload the shortest position. Here is a great variation to really target the lateral head of your triceps!