A lot of accounts have higher trust rating now, how many different positive trust feedbacks does it have? How long ago can you sign a message from associated with this account as you have not stated and that is a big deal. I have to give you a heads up for a newly Hero account you are not getting nowhere near 3btc! Sorry to break it to you, you will be very very lucky to get 1btc 
Thank you for your answer
Honestly I'm not very skilled in cryptography, how can I sign a message and why it's so important?
Signing a message shows that it is in fact your account, and not just hacked or otherwise stolen. But you really should reconsider selling this account; the positive trust is likely to be used in scams - there aren't many reasons for someone to bother paying more otherwise. Alternatively, consider placing a neutral trust warning in the account, advising users the account was transferred to someone else; this will, of course, lower your profits somewhat but, as blumangroup mentioned, you aren't going to get anywhere near the amount you're asking anyway.