Maybe trust the word of someone who flies the aircraft type the OP was asking about. The use of Medium autobrake is expressly not recommended with Config 3. Good luck making A9 or N5 at Heathrow with Config 3 and Low Autobrake. Incidentally Munich is one place where Config 3 and Low Autobrake could work quite nicely.
But maybe the OP wants answers from 747 drivers transposing their SOP’s onto another aircraft of which they are either unfamiliar or have no recent experience with.