Well if napoli win by 3 they'd have even GD, does then goals for rule then apply?
If dortmund and napoli both win then Arsenal, Dortmund and Napoli will all be on 12 points. They will be split by the results between the three treams in a mini league. In the above scenario all would have the same points (as everyone took 6 points from marseille) so it would go down to goal difference again.
At the moment Dortmund are on +1 (this can't change as they are playing marseille which doesn't count in the mini league) Arsenal are on +2, and Napoli on -3. So Napoli need to beat arsenal by 3 goals.
Simples