How could sulfur be oxidized to #SO_3# by the action of #NO_3^(-)#?
Sulfur is oxidized to sulfur trioxide. Nitric acid is reduced to nitric oxide,
Zerovalent sulfur is oxidized to
This is balanced with respect to mass and charge. It would not be a feasible reaction in practical terms. Industrially, sulfur trioxide is produced from sulfur dioxide and oxygen directly with some form of supported catalysis. It must be an incredible dirty and smelly process.