Never been through the NATS process, but in the absence of any other info, it sounds like a logic test. The 74 burns the most gas, is most expensive to operate, it goes first, then the 'bus, then the 172.(The 172 will have to wait at least 4 min for wake alone before it gets a shot, in that time it will use maybe $2 of gas.Nothing.)
That (I think) should be the basic answer. Variations can then be added. As the others have said, there's more than one "right" answer, depending on circumstances. Eg if the Cessna can be departed from, say, a crossing runway without delaying the other two, let it go first. If the 'bus has the King og Narnia on board, and has requested priority, it goes first. If the 'bus's destination is in line with a crossing runway, it can go first off that runway if it doesn't delay the Boeing etc etc.