I believe the questions that need to be answered are :
1) Is it confirmed the 3 aircrafts actually declared a mayday fuel or did they opt for a minimum fuel / PAN call and ATC has asked them to squawk 7700 for their own London Area Ops ?
2) In case of a multiple mayday call by 3 aircrafts diverting, what is the root cause ?
Being clear on the above could be helpful.