Not only... but also

Consider the following:

1. Not only you should be able to speak but also able to write.
2. You should be able to not only speak but also write.
3. You should not only be able to speak but also be able to write.

Which is right to use? If there's an even better one, please suggest.

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.