Imagine you have three people with fruit that is to be handed out to a group. Everybody has said beforehand what they'd like most, 2nd and least.
All at the same time, each person hands a list to the fruit distributors who should be offered their apples, their oranges or their bananas. The distributors look and see Fred who has put down apples as his 1st preference is top of the list. Fred is offered an apple. If Fred accepts it, he then walks away with his apple. Daphne, who didn't particularly want an apple, is next on the list of both apples and oranges. Because she prefers oranges and she's top of the orange list, she gets offered an orange. Vilma wanted bananas, even though she would have got an apple or an orange if she wanted one, so she gets a banana.
This continues until maybe the apples are all gone. For some people, they would have preferred an apple, but they get offered their second choice of an orange. Some accept happily, others accept but say they'd like to be offered an apple if anybody comes back and says they've changed their mind or isn't hungry after all. Others ask for a reconsideration because they've decided they do not want their second/third choice fruit. This happens until everybody has been offered fruit or all the apples, bananas and oranges are gone.
Unfortunately for Shaggy, the fruit he preferred has all gone. However, the fruit distributors say 'We've got some strawberries, would you like those instead?'. Shaggy either accepts, hangs around on the offchance that somebody announces that actually, they've decided to go and buy ice cream so they don't need fruit after all and returns theirs, or asks for it to be reconsidered/if somebody could check out back to see if there's another apple.
Scoobie, though, says if he can't have apples, he's not going to accept anything else, thinking that'll make the apple person give them an apple. As nobody has returned their apple, Scoobie then finds himself without any fruit at all and has to find some himself after everybody else has taken their preferred fruit.
You put your favourite school 1st, your next 2nd and your third, 3rd. If all goes well, you'll get 1 or 2, maybe 3. If not and assuming there's nowhere else further away with spaces/it's not equally oversubscribed and the majority haven't already qualified for an offer at your 1, 2, or 3, the chances are that you'll be offered the one you haven't included.