phrases - Not only... but also

The second and third sentences are correct:

• You should be able to not only speak[,] but also write.


• You should not only be able to speak[,] but also be able to write.

I would correct the first sentence thus:

• Not only should you be able to speak[,] but also should you be able to write. OR


• Not only should you be able to speak[,] but you should also be able to write.

I leave it to you to decide whether you want a comma before but. Either way is fine.