I would actually say that none of them are 100% correct. Grammatically, #1 is incorrect. #2 and #3 are grammatically correct but you don't "agree" with an offer. You "accept" an offer. So, perhaps what you want to say is, "It is hard to accept your offer."

Both sentences 1 and 2 are correct and are equally common. The first sentence would not be used because there is no preposition word ("with") that establishes that it is the offer that is being agreed upon.

I think 1 could have been correct if you had written it in this way: your offer is hard to be agreed with

English people don't say "to agree with an offer", it's a Russian phrase in English it's "to accept / refuse an offer"