I believe the numbers you are looking for are contained within the published results at:
http://www.sfgov.org/site/uploadedfiles/election/results.htm
I've seen the round-by-round result on the SF web site, but I believe you'd have to crunch through the ballots to know exactly how many of the exhausted ballots contained less than three choices. Of course we'll never know how many voters would have ranked more than three.
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