Calendar, Notification when highlighting 3 dates at the same time

Hello,

attached is an excel file as an example. calendarTEMP.xlsx

Right now I'm updating the annual calendar for a company. Only 2 of the 3 supervisors are allowed to take or highlight their annual leave on the same date.

I need a notification popping up or a lock, when the 3rd supervisor tries to highlight his annual leave on the same date as the other two (for example: see file: Supervisor C is not allowed to put in his annual leave on Dec 3,4)

Can you help me ?