As a layperson, you don't need to worry about that.
IIRC we have discussed this topic here before. Even contracts concluded in front of a notary can be disadvantageous for one party or another, so the standard forum advice applies here as well: Have it checked! (Who should check depends on the contract.)
Still, regarding the initial question.
There are 3 heirs who want to sell it together and are supposed to agree
Well, ideally the three heirs (!) agree before they go to the notary. They agree on what they actually want. Once they have set the goal, they go to a notary of their choice and ask for advice (!) on the matter. Afterwards, they will surely be wiser, and with specific questions one can always turn to the public of the internet ;-).