Read the FAQs. There is in some scenarios that the top three preferences are considered equally in one round.
QUOTE FROM UAC:
"Are all my preferences given equal consideration?
Yes. If you're not selected for your first preference, you'll be considered equally with all other eligible applicants for your second preference and so on. Your chance of being selected for a particular course isn't decreased because you placed a course as a lower order preference. You won't be selected for a course just because you entered that course as a higher order preference.
For example: Anna doesn't qualify for an offer to the courses she has listed as preferences 1, 2 or 3, and has course 116677 as preference 4. Anna is eligible and more competitive than Blair and Catriona, who have both listed course 116677 as their first preference. Anna, therefore, receives an offer ahead of Blair and Catriona."
So depending on what type of student you are... Good Luck in December, Early Jan and Main Round... and note sometimes only first preference in considered in some rounds so read the UAC FAQs/Guidelines very carefully.