Actually they rotate counter-clockwise. The 3b umpire today will be at hp tomorrow.
For 2nd , 3rd , and 4th games of a series you can check previous day's boxscore and then rotate counter clockwise to know where they will be.
For 1st game of a series you'll need to subscribe to sports options premium odds service to get notification. (they're an rx sponsor btw)
If you dont want to pay you can get them from the mlb web site , but they are not posted until line-up cards are exchanged , usually about 30 mins before gametime.