score:4
Yes, that's about right. 3 hours sounds eminently doable; it's not a large airport and most passengers transit anyway, so there are not huge security lines.
Of course, if your incoming flight from London is delayed and you don't make it, you'll be on your own, with your onwards ticket lost as a no-show. Flights to Narsarsuaq are only twice a week, and possibly expensive on short notice -- or may even be sold out, as the August 12 and 15 departures currently are -- so if it were me, I think I would plan to arrive in Iceland the day before, just in case.