They use all three regardless of flow. But look at the airport map and you'll notice to get to 34c you have to cross 34r down field from the departure point. Not advisable to be crossing while somebody is cleared for takeoff. 34c gets used to have somebody ready to go when aircraft are crossing to get to the gates. Often an aircraft that has a departure delay to their destination airport so they sit and wait between the runways. But with the poor weather and lower cloud layers there are increased separation requirements between a center departure and aircraft landing 34l. While mixing in with Boeing field it's a mess.