Can/could/will you slow it down

"moving fast"/"slow it down" are not very idiomatic for a fan. You might say "The fan is too strong. Can/could/will you turn it down a bit?".

"could" is nominally more polite/tentative than "can", but without "please" at the end none of the options are especially polite (though they needn't be rude). "Will" is more like a command than a request. But in reality, the politenes

AbilThe fan is moving fast. Can/could/will you slow it down a bit? I cannot stand cold.

Is the above correct? Which of the three choices would you suggest?

I would have written it differently:
The fan is blowing too hard. or The fan is set too high.
Can you turn it down a bit?
The cold bothers me.

Mr WordyIt's nominally polite, but you can say it in such a way as to make it sound as if someone's obliged to do as you ask.

This interests me. Could you give me a scenario, Mr. Wordy? Thanks in advance.

Can you turn it DOWN, PLEASE

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Capitals show the stress; graph shows the pitch.

The site is not good at aligning dashes for some reason. I happened to me a few days ago. But I think I get the idea though I'm interested in knowing the pitch. Don't worry about it since it's quite unlikely to fix the problem.
Thanks, Mr. Wordy.

New2grammarThe site is not good at aligning dashes for some reason.

I set it to fixed-width font in the HTML editor. It was fine in the edit pane, but now I see that the stoopid system just overrides it in the normal display. If it's not obvious, you can copy and paste the whole thing into any text editor that can show a fixed-width font (such as Courier).

Thanks. I have learnt a lot.