Of course, Heathrow once had six runways (numbered 1, 2, 4, 5, 6, 7 on the map I saw, so even then they didn't have runway #3).
I thought the justification for the third runway was to let a lot of the smaller aircraft use it and save them spending time burning fuel in the hold. No doubt they'd end up at capacity again with three runways and the same amount of delay though.