Gear Down 3 Greens then Flap 15
I can think of a couple of reasons for waiting until the Gear is indicating Down 3 Greens before selecting flap. Primarily, it makes sense to confirm the action taken results in the desired outcome. In this case, Gear selected Down - Confirmed Down 3 Greens. Select desired flap setting. In this case, Flap 15 selected - Flap 15 indicated. As previously posted, it makes sense to make the selection of Flaps 15 after the Gear is down 3 Greens, in the event a hydraulic malfunction occurs, to make it easier to identify which system is at fault. From a systems stand point it is also prudent to not overload the hydraulic system/s with both Gear and Flap running at the same time if they are both powered by the same system. Besides, what is the rush? Surely the aircraft is configured in ample time to preclude undue haste!!