If 3rd runway was built then I think capacity for the three runways combined would be capped so as to provide more resilience than the present declared capacity for the two runway operation which have very little resilience should an unplanned runway closure occur or low visibility procedures are put in place.
I read somewhere recently that Heathrow Airport have indicated that it would not permit scheduled movements to land before 0530hrs local should a 3rd runway be approved. I understand that presently it is not before 0430hrs. Whilst this may give noise relief for an extra hour to those under the arrival flight path it's unlikely to affect the number of arrivals before the 0600hrs end of curfew. Heathrow's present night movement quota limits movements between 2330-0600hrs to an average of 15 to 16 per night. Virtually all are arrivals and which can be achieved for the 30min period between 0530 and 0600hrs, so this noise relief offering is likely to have little or no effect on Heathrow's business.