How are final pairings determined in the PGA with a 3 way tie going in to the final round?

Question

noticing that mickelson and singh were tied most of the day but fell back and tied with westwood.

Answer 1

Those with the same score finishing first will be the last pairing. The third will be assigned to the second to last pairing.

Answer 2

Deadsolid got it right.  The pairings are set by the order in which the scores are posted.  So the first one at the low 54 hole score is the last player, the next to post that same score is their playing partner, the next is in the group ahead of the last pairing.  This gives incentive to score low and score low quickly.

Answer 3

their rankings

Answer 4

If there is a tie or for all pairings, the first scores posted go out last.