Question

Is having 3 dark types in your team a good idea?

Answer 1

Usually, no. Besides in a mono dark team, having 3 of the same type will exploit the same weaknesses to the team, making Fighting and Bug types a problem.

Answer 2

Unless two of them are Spiritomb and Sableye, that's a negative. Fighting type moves are present on almost every moveset of every Pokemon in competitive battling. the only way to justify the use of 3 Dark types outside a MonoDark team is Spiritomb and Sableye being Ghost/Dark types, which leaves no weakness. if that's not the case, change your line up.