Can I vote for only one person as a moderator (i.e. no second/third choices) or should I use all three votes for it to be counted? Are there online hanging chads?
I don't think this answers your question (since the explicit implementation of the voting system depends on the use of OpenSTV for vote counting, so it depends on whether incomplete rankings are explicitly allowed in the electrion), but you should take a look at the theory behind Single Transferable Vote: to pedantically correct a statement in your question, you don't have three votes. You have only one vote on which you specify a ranking of your preferred candidates.
present case where there is only one winner ( correct me if I am wrongEdit: I am wrong indeed. Thanks Grigory M.), your second and third choices will not be considered at all if you happened to vote for either of the winner or the runner up (since the lower choices are only used in this case when your candidate is eliminated from the election, and neither of those two would be), which, if you choose randomly, is a fifty percent chance (considering we have four candidates). Furthermore, if the community has a clear favourite, none of the candidates would be eliminated (or perhaps only one), and again, unless your top choice has been eliminated, your second choice won't come in effect.
This is just to say that more likely than not,
considering we are if we were using STV on a four candidate pool for one position, your second and third votes won't matter anyway. So even in the case where the software platform does not allow incomplete rankings, you can be comfortable knowing that a random ranking of your second and third choices likely won't affect the final result.
(Lastly, a bit of philosophy: STV allows you to rank the candidates. Whether you do it by most good or least bad is up to you. Even if you have absolutely no preference between the three other candidates, it terms of the election result it wouldn't hurt your preferred candidate to randomly specify the lower rankings; nor would it help to not specify a second/third choice [your vote will just be "wasted" if your top choice were eliminated]. So I say just go ahead and fill in the second and third choices!)