I found in Oxford Dictionary that discount word can be used with two preposition:

discount on sth.


discount off sth.

I found different using of this word on the internet:

1. 10% discount off of the price
2. 10% discount off the price
3. 10% discount of the price

So, what is correct form for using discount?

All seem fine to me. Perhaps #3 is less usual.
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#1 is horrible IMO. "off of" is always horrible, in fact.

#3 is not correct in my usage.

It is usually obvious that a discount refers to price, so you don't usually need to say so. I suspect that the "sth" in the dictionary guideline refers to the item for sale (e.g. "10% discount on suits"), which is rather different from your examples.
I think we should make it clear that there is a difference between 'horrible' and 'informal', which is how 'off of' is listed in dictionaries.