I didn't see anywhere that it specifies whether or not we have to use all of our votes in elections. I know that process for determining winners is STV, under which it is possible to include voters who didn't use all of their votes, but I just wanted to verify that my votes would still be used.
Yes, your votes will still be used. You may cast just one or just two out of the three you are allowed.
I base this conclusion on having analyzed (in detail) the results of some other elections. A close look at some of those data summaries and visualizations will show that some voters cast incomplete ballots but their votes contributed to the results.
Just to confirm -- votes that had only one first choice, or only a first and second choice were present in the election that was held ... and were used in the way you would hope.
Here's a little piece out of the (publicly available) data file, with spaces added, some column headings not present in the original (for context) and some added comments:
num 1st 2nd 3rd terminal votes choice ch. ch. zero ... 1 1 2 3 0 # voted for candidate 1,2 and 3 in that order 1 2 3 4 0 # voted for candidate 2,3 and 4 in that order 1 1 0 0 0 # only voted for candidate 1 (A) 1 1 0 0 0 1 1 2 0 0 # only voted for candidates 1 and 2 (B) 1 1 2 4 0 1 1 3 2 0 ...
Who is "candidate 1" and so on is given by the order of the names near the end of the vote file.
The first column is always 1 for SE elections (identical votes are not aggregated, but the file format supports such aggregation), and the last column is always 0.
That these were used can be seen by the
Exhausted vote in round 2 here. That's the votes like
the one I marked "(A)" contributing 38% of their vote toward electing one moderator, but then wasting the remainder of their vote, by then failing to express any opinion about the later candidates in respect of the second open position. While it's possible some of them might have felt there was no way to distinguish the later candidates' suitability, I suspect the main cause for many of those is simply lack of understanding of how single transferrable voting (STV) works.
People who voted "1 2 0" or something like that (such as "(B)" in the above) used more of their vote. There was no third round (but it was possible there could have been one), so the absence of their third choice had no impact at all, and as it happened, no votes other than "1 0 0" were prematurely exhausted.
They all counted as they should for an STV election.