If doing a 3rd away means that you will be tired and not on your A-game for the last away, then it can hurt you more than help. If you will be just as awesome on your 3rd away as your previous ones, then its usually advisable to do the away. However a couple of questions come to mind:
1) Is this 3rd away at a program that is a reach for you or is it more within your reach judging from the caliber of students who have matched in the past? If its your reach or dream program, you might not want to save it for last to have the most energy left it.
2) Is this 3rd away at a program that is in the same region as your others. I.E.- if you are trying to get a good LOR from this last away, lets say UPenn, however you've also rotated at Temple, Drexel, etc, then you've saturated the area and that last away may not really help.
I could really go on and on but my overarching themes are don't do the away if you will be too tired or if the LOR won't really change your overall application.
