they will look at all of the children who have applied for spaces, thertes a possibility that if this a popular school, then other parents will have applied who are not in catchment, they will assign all the children in catchment, a space, then any extras will be assigned on other criteria, ie, distance from the school to home, any brothers or sisters currently at the school etc, they may be oversubscribed by 3 but could have say 5 who arent in catchment area and who live further away from the school than you do.