Originally Posted by
procede
You will need to alternate (technically not mixed mode, as you are not using the gap between two arrivals for a departure) one runway to balance daily capacity if you have three runways.
I don't understand the distinction you seem to be trying to make. Interleaving arrivals with departures is exactly how one of the three runways would be used for most of the time. That's mixed mode in anybody's book.
It also works better with arrival and departure peaks (AMS uses this). As a departure runway has more capacity than one used for arrivals, departure mode will need to be used less often than arrival mode.
Other than in the early morning and late evening, departure and arrival demand at Heathrow is pretty evenly matched. That, of course, is how the airport currently operates with its two runways, one designated as the landing runway and one for departures throughout most of the day (though on most days there are some arriving aircraft that land on the departure runway)