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Say we have three candidates: A, B, and C.

Say, a voter wants to vote for C. However, he knows that C can’t win and hence choose A instead.
Hence, in a sense, the voter is “dishonest”. He doesn’t pick his most preferred candidate but strategically chooses the preferred outcome.

What would be the term for that?
I looked for voters dishonesty on Google and couldn’t find it.

It’s called tactical voting.

From Wikipedia:

In voting methods, tactical voting (or strategic voting or sophisticated voting or insincere voting) occurs, in elections with more than two candidates, when a voter supports another candidate more strongly than their sincere preference in order to prevent an undesirable outcome.

As Andrew Grimm correctly pointed out it is tactical voting you are looking for. However, I would avoid harsh terms such as dishonest since Wikipedia also mentioned that:

It has been shown by the Gibbard–Satterthwaite theorem that any
single-winner ranked voting method which is not dictatorial must be
susceptible to tactical voting

More details are provided by the dedicated Wikipedia page:

(..) with deterministic ordinal electoral systems that choose a single
winner. It states that for every voting rule, one of the following
three things must hold:

• The rule is dictatorial, i.e. there exists a distinguished voter who can choose the winner; or


• The rule limits the possible outcomes to two alternatives only; or


• The rule is susceptible to tactical voting: in certain conditions some voter's sincere ballot may not defend their opinion best.

In a game theory context, this is kingmaking. The player's goal is to elect C, but once the player starts to lose, the voter influences the outcome of the game (in this case the election).

This is a common problem with three-player and beyond games. For example, if I were playing UNO and I was losing badly (I had maybe half the deck in my hand), I could start to always change the color to, say, red.

The problem arises when players' ability to influence the game does not vary directly with their distance from their goals. In some games, the closer you are to losing, the less ability to affect the eventual winner you have, like Monopoly (which has a whole host of other problems of its own, but we'll ignore that).

Another example would be in many kinds of online games, where you might realise your character doesn't have enough time left to reach some checkpoint, so you go try to knock other characters around and prevent them from reaching the checkpoint.

Dishonesty isn't really the best term. More like 'choosing the lesser of two evils'.

It happens all the time. No candidate is likely to meet all of an individual's criteria, so they choose the one that meets the most.

On the other hand, if more people would choose candidate C, they might win. Or, they might draw enough attention to mount a serious challenge in the next election.