Places table:
Name(key)
Address
Capacity

Events table:
Name(key)
Date
Place

Orders table:
Id(key)
EventName
Qty

string eventName = "Event";

var theEvent = Events.FirstOrDefault(ev => ev.Name == eventName);
int eventOrderNo = Orders.Count(or => or.EventName == eventName);

var thePlace = Places.FirstOrDefault(pl => pl.Name == theEvent.Place);
int ticketsLeft = thePlace.Capacity - eventOrderNo;

int placesCapacity = Places.Where(pl => pl.Name == theEvent.Place)
                           .Sum(pl => pl.Capacity);

int ticketsLeft = placesCapacity - eventOrderNo;