What are the differences among these three sentences?
- Who would most likely be listening to this talk?
- Who will most likely be listening to this talk?
- Who is most likely listening to this talk?
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Let's pick these sentences apart. Let's discard, for now, the adverbial "most likely"; it is common between all three of the sentences and plays the same role in all three. This leaves us:
Now, the third example is simply a normal interrogative in the present progressive. It uses to be in the normal way as an auxliliary. The second example is exactly the same but in the future, using the auxiliary will applied to to be.
Would has a few uses. It is generally thought of as the past tense of will, so it can be used for future-in-past (e.g. "we did not know that he would go on to be a great physicist"). It can also be used to express preference ("I would rather have a beer"), and to talk about hypotheticals ("If you asked me out, I would probably faint").
In this case, it's worth bringing back in the most likely adverbial, and see that this also indicates a hypothetical, so it is probably a hypothetical - talking about an imagined scenario, such as:
"Let us imagine that a great mathematician came to the university to give a talk on their research. Who would most likely be listening to this talk?"
So, here the third example is a question about a talk that is actually happening (though we sometimes use indicatives to discuss hypotheticals, which confused matters). The second is about a talk that is actually going to happen. The first is talking about a hypothetical talk. In all cases, it is assumed that the listener, and possibly the speaker, don't know who is, will be, or would be at such a talk, as they are assessing what is likely to be the case.