Please note for the hole 3mm you need to drill just a little large to receive the head of countersunk screw.
It depends of the diameter of the head of your countersunk screw. For me it is 5,5mm. The head of the countersunk must not flush from the adapter plate.
Do you mean the head of the screw must be flush or below the surface of the adapter plate?
Pat, can you have a couple of these ready for me at York?