Reservations for Three Broomsticks are only for the Breakfast that is currently offered only to HP package holders. You chose the day and time when you purchase the package.
Otherwise, Three Broomsticks is a Counter Service restaurant and you don't make reservations. Currently, I believe they have a plan to handle the heavy summer crowds. According to TMs who went to the preview last week, you will line up to enter. A TM will ask if all your party is present and paying together. You will be directed to a particular register to order your food. When you have your food another TM will direct you to a seating section/or your table. There will be NO table holding (YAY) or dwindling allowed and no waiting an hour for the rest of your party to get off a ride (that's what the owlery is for).
That's always been one of my biggest theme park pet peeves, not being able to find a table because someone is saving three for a party that's not there yet.