If only us and Napoli will be tied on points yes, the +3 is fine so worst case we lost and Napoli win then we win on H2H goal difference.
If anyone else was tied on points (Juventus, Atalanta, and/or Lazio) then H2H is based on H2H points taken from games against any tied-points teams I think? In that case, Lazio have H2H over us, and we'd need to win both Juve and Atalanta game with a better GD to have the better of them. So, we really need to hope they drop more points or we beat both at this point. For example, Atalanta finish second and then if us, Napoli and Juve all tied on points and say we draw to Juventus next week then we've taken 1W 1D 2L (4pts) from those two teams, while Napoli is 2W 2L (6pts) and Juve would be 2W 1D 1L (7pts) so we would not get top 4. At least that's how I think it works?