Which on is OK?
I'm sorry I can't be more help.
I'm sorry I can't be of more help.
I'm sorry I can't be more helpful.

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All three are good. I wouldn’t split hairs on the differences. To do so is just a waste of time.

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The first one is not correct!The second one sounds good and it is acceptable to be used in a sentence, but the third one sounds more natural and it is more commonly used!Best option: "I'm sorry I can't be more helpful"Hope that helps!

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Even though the third one sounds more natural, the second one is also acceptable.

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I believe third one is better than others.