No there aren't three Triceps there's three Tricep "Heads"

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You have the Medial, Lateral and Long.
I don't think you can actually work them out individually. you would probably hit at least two at once.
The only way I can think of offhand, would be to perfform an exercise like Kickbacks and use a half reps movement, a la Joe Weider, which might actively cut out one of the Heads from being stimulated.
Apart from that I don't realistially think you can anyway.
Plus if you want to cause extra exertion on Triceps, don't be too keen to opt for Isolation stuff a lot like Pressdowns, Kickbacks, Extensions etc etc.
Dips will work, but also so will things like DB Bench Press, or Military Press as examples, as any pressing movement will have some secondary effect on the Triceps.
So I don't see it as a problem, to incorporate an Isolation move in the mix, but don't go for them as a major bulking method, use some Compound stuff to work them as well.
A good body comes from hard work.