Should the "command" method do the validation?

Should validation in a domain Entity be implemented as separate method or as part of the Entity state changing method?
 

 For example if I have event booking application where each event has limited number of places for participants. I want to check if user can make the booking or not.
 

 Variation 1:
 class Event {
   Result add(Booking booking) {
     if (fits(booking) {
       this.bookings.add(booking)
       return Result.success()
     }
     
     return Result.fail(NOT_ENOUGHT_PLACES)
   }  
 }
 

 Variation 2:
 class Event {
   boolean fits(Booking booking) {
   ...
   }

   void add(Booking booking) {
     bookings.add(booking)
   }
 }
 

 Variation 3:
 class Event {
   boolean fits(Booking booking) {
     ...
   }

   void add(Booking booking) {
     if (!fits(booking) {
        throw ConstraintViloationException("Cannot fit booking")
     }
     bookings.add(booking)
   }
 }
 

 Variation 1 seems nice but violates the Command and Query separation principle. Variation 2 is fragile because what if client forgets to call the fits() method. Variation 3 seems like duplicating the same check.