Just throwin this out there, what does you being a hardgainer have anything to do with what tricep exercises are best???? And imo the best exercise for triceps are weighted dips on parallel bars.
I know one that'll make your triceps hurt like hell by the end of the second set. Tricep pulldowns (on the cable machine with two-hand attachment) 4x10 In between these sets, do free-weight dips on a parallel bar, as many as possible. Repeat till sets are done, you'll be pushing your pulldown weight RIGHT down by the end of it.
All these exercises are perfect, but try combining them into a superset. If you're not sure what that is, it's when you do 2-3 exercises consecutively with no rest in between.