2nd in command to sauron

[Gwaihir the Windlord]

Witch King, I believe he was the chief of the Nazgul but i guess in Mordor's chain of command he was 3IC.

I do not think that the chain of command was quite as rigidly hierarchical as this. The Witch-King would certainly not take orders from the Mouth of Sauron; he answered to the Dark Lord himself.

As the name implies, the Mouth of Sauron was Sauron's spokesman. The Witch-King was both the commander of armies, and of the 'elite special forces' group that was the Nazgul. These positions are not interchangable. The Mouth was to be Sauron's 'spokesman' -- that is to say, his ambassador or representative -- in the lands that he was about to conquer.

I'm afraid I'm going to have to correct you, Eurytus:

Now at that time the Chieftan of the Ringwraiths dwelt in Minas Morgul with six companions, while the second to the Chief, Khamul the Shadow of the East, abode in Dol Guldor as Sauron's lieutenant, with one other as his messenger.