If you listen to Edge of Sanity unorthodox there is a 3rd guitar in the middle of the right and left guitars always. All the guitars harmonize sometimes causing notes to sound really really low.
A lot of older death metal albums have a 3rd guitar. Not always but at specific moments. Bloodbath is a new band that has a lot of third guitar parts.
On a cd if a right or left guitar cut in and out of rythmn to do harmonies and solos it would sound more like a live band. I think studio recordings are suppose to be more clinical. You rarely hear a left or right channel guitar on metal album playing solos for example. Almost always a third guitar.