We are given three votes to cast in the moderator election. What happens if one casts only one or two votes?
And how does such behavior plays within the STV system used?
We are electing two moderators. If your choices #1 and #2 get elected, then your choice #3 is irrelevant, it's the same as if you did not make it.
If both of your choices #1 and #2 are eliminated midway through the STV process, then
If one of your top two top choices is elected, while the other is eliminated, then the elected one gets to keep a large % of your vote (whatever they need to be elected), while the rest gets treated as above.
My conclusion: even if you have a weak preference for #3, it's still better to express it than to (essentially) abstain in a situation when 1 and 2 did not work.
This is an excellent question. The short answer is no, you don't have to cast all three votes. However, if there are multiple candidates that you like, then it is a good idea to cast multiple votes. Casting a second or third vote does not in any way lower the value of earlier votes.
The election uses the Meek Single Transferable Vote system. This system is designed to alleviate some of the problems that come from the prototypical one-person, one-vote, winner-take-all system (sometimes called first past the post).
For an excellent description of this voting system, I recommend reading this answer from Jon Ericson, which includes some examples within.