The whole story is so very bizarre.
Assuming there have been three separate sightings of the drones it is obviously a deliberate move. One of them was apparently only 45 minutes after the runway re-opened at 3.15 am, so if the drone operator was able to react that quickly and at that time in the morning, they are clearly moving with intent.
The possible options I can think of:
1) Relatively harmless but disruptive:
Environmental or anti-third runway protestor - who, given the massive outrage from their actions, has decided to keep quiet.
Prankster - maybe once, but coming back twice more, including daylight when there is a large police presence.
Youtuber hoping to film a military response shooting down their drone - but surely they would do this during the day?
2) Dangerous:
I know that police have apparently ruled out terrorism, but I'm not sure how at this stage, unless they know a lot more than they are letting on, or are just trying to avoid panic.
In Syria, Russian airbases were attacked earlier this year by explosive laden drones which caused damage to several aircraft and has been blamed on IS terrorists. Managing to hit an aircraft in flight with an explosive laden drone would have horrific consequences.
Even without access to explosives, it would be pretty easy to make a large drone a really dangerous target - the drone alone could cause major problems for a small aircraft - put a container of flammable liquid or ink on it and you have a highly serious incident right there.
Of course the other option is mass hysteria. As mentioned above, we have no images of these drones and twice they were seen at night. Could the first sighting have been real, followed by later mistaken identity sightings by jumpy security? Could even the first sighting have been mistaken - it was after 9pm at night and at the airfield boundary? Was it phoned in (maybe by a prankster) or seen by an aircrew or security officer?