There's no real reason why voting for the same candidate three times should invalidate your ballot and it wouldn't help his or her chances either.
Remember that second (and third) votes are only counted once your first choice has been eliminated from the election so if you choose the same person as a second and third choice you will be voting for someone already out of the contest.
We use a similar system here (no.2 below) where you number as many of the candidates as you want, all of them if you like - if there are twenty you can put '1' as your first choice down to '20' for last. Candidates are removed (by lowest total) one-by-one until someone gains a majority of over 50% and only your first choice is ever considered until that has been eliminated.
