Breakfast at 3 Broomsticks is offered to those on the Harry Potter vacation package, and is done by reservation. However, if they are not overbooked with reservations you can usually get seated by asking the person stationed outside. I have also heard of people having success getting a reservation while not on the package by asking at the concierge at one of the onsite hotels although I have never tried that.
I will warn you that if you are not on the package(where the bfast is included in your package price) that it is quite expensive for counter service eggs ect. While there for the Film Celebration where we were let in at 6 AM they were allowing us to eat without the package or reservations and for one of the breakfast platters which includes a small drink it was $16 a person. I ordered the English Breakfast and added a coffee and the total was $19.50. There are no prices on the Breakfast menu as it is a set price meal since it usually paid for by a voucher they give you with your package.