Willow, read about my discography here:
My c-spine acting up again... (discography results)
If there are obvious structural problems at 3 levels, especially with spinal cord involvement, then there is no need to do discography on them. There is no "take it or leave it" decision to be made... they are going even if they are not painful. However, you can have these structural issues that need to be addressed, and they may not be your pain generators. If there is a disc next to the obviously needing replacement levels that is bad, discography there will keep them from leaving a disc that is causing substantial symptoms.
This is not all black and white and discography is not perfect, but it is a useful tool to help YOU to make informed decisions.
Mark
PS... obviously, this is all hypothetical... not specifically about your case.