I get asked often what I do to train my triceps. My two favorite exercises for them are the lying close-grip behind the head tricep extension (like skullcrushers but with a different range of motion back behind the head rather than to the forehead) and weighted tricep dips, aka weighted bench dips.
A lot of people get hung up on keeping a portion of the arm stationary while another if free to move only a certain range, blah, blah, blah. You can decide what works for your body and what range of motion hits your triceps! Every body is different, adapt your training to your body.
Here is a video for the extensions