How does voting work?
There are three votes, but the votes are ranked.
You select a first choice, a second choice, and a third choice. (It is possible to only make a first or a only a first and second choice.) You do not cast three equivalent votes.
Once voting ends, initially only the first choices are considered. The candidate that comes in last is eliminated, and their ballots are moved to each voter's second choice. Votes are counted again, the new last place candidate is eliminated, and once again and their ballots are moved to each voter's next best choice. This process continues until a winner is determined. For details, see How are moderator election votes counted, in plain English?
In short, as long as your first choice has not been eliminated, your vote only counts for them. Only if they are eliminated does your second choice become relevant.
Therefore, even if you have a clear preference for one candidate, you still should consider making a second and third choice if you have any preference between the remaining candidates. It will not harm your first choice at the start (your vote only counts for them), and if they do not make it, you still retain some influence in the process.
Can picking 2nd/3rd choices reduce my 1st choice candidate's chance to win?
No. Your 2nd choice does not activate until your 1st choice has already lost definitively, so picking a 2nd choice cannot hurt your 1st choice. Similarly, picking a 3rd choice candidate cannot hurt your 2nd choice's chances.
Picking only a first choice candidate will not make your vote for them any "stronger."
What is the effect of only picking a first choice candidate?
The only effect this would have would be to abdicate your ability to express preference between remaining candidates in the case that your first candidate does not win.
You should only do this if you like one candidate and equally dislike all other candidates.