As far as I know, most references to invertebral discs would be by the two vertebrae it lies between.
eg: "...herniation of the L4-L5 disc...."
I have not heard of the discs themselves being numbered; but, with the numbering sequence you mention, I would take a guess that the discs are numbered the same way as the vertebrae---so, T3/T4/T5/T6.