Proper configuration
Hello to all , please remember the proper position of the LE Slats with the trailing edge flaps position , so with the classics and the standard NG the answer is YES you should wait for three greens because it is an improper configuration to move your LE Slats to landing configuration ( full extended ) before you have the three greens that means down and locked , and the reason is that with the classics and the standard NG your LE Slats move to Full extended position with trailing edge flaps passing 10 , that is why if your airline is using flight data monitering or FOQA it will trigger an alarm of improper landing configuration with the classics and the standard NG .NOW it is different with the SFP because your LE Slats stay at extended position till 25 , then passing 25 they move to Full extended position for the proper landing configuration , so the answer is NO you needn't wait for three greens to set flaps to 15 with the 737 SFP ( although with mixed fleet the airline FDM or FOQA will still trigger an alarm but technically and aerodynamically you are OK and highly justified for your action ) .
Best Regards .