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I know that the following three sentences could be used to mean the same, but what would be the order of (your) preference? I wouldn't say [2] or [3] for the reason I don't know, but somehow [2] and [3] don't click. You would probably prefer "I doubt if you can" to "I doubt that you can." Hence, would the order here be [1] > [3] > [2]?

[1] I'm not sure if you can.
[2] I'm not sure that you can.
[3] I'm not sure you can.
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All seem fine and common to me. Also 'whether'.
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I've said these to myself several times, and I can't say that I have any strong preferences at all.

CJ