Three runways are useful as two can be used for departures and one for arrivals at 'departure peaks' and two can be used for arrivals and one for departures at 'arrival peaks'.Three runways are useful as two can be used for departures and one for arrivals at 'departure peaks' and two can be used for arrivals and one for departures at 'arrival peaks'.
Other than for an hour or so in the early morning and an hour in the late evening, Heathrow doesn't have "departure peaks" and "arrival peaks", in fact it doesn't really have peaks and troughs at all (hence the 98% utilisation).
The Airport Commission's two shortlisted proposals for Heathrow both involve balanced arrival and departure capacity, achieved by operating one of the three runways in mixed mode for the majority of the day.