Not always easy to answer that. It depends on the particular signaller/crossing keeper on duty at the location that supervises the crossing. However, in my experience, with so many trains a three minute separation would probably not be sufficient to raise the barriers. Management always wield the big stick of performance at signallers for delaying trains. The crossing sequence will require about 20-30 seconds to lower, and after a train about 10-15 seconds to raise. A train will travel a considerable distance in that time, and the driver would be expecting to see proceed aspects, unless of course they are closely following another service. A motorist sat waiting may lose perception of time when they see one train pass and another is already approaching. I've had people complain to me they've been sat waiting for "20 minutes" whilst two train pass the crossing, when in reality it's probably been no more than about 2-3 minutes. The speed of a train will also have an effect on things.