3 - I recommend "It's best not to know." This alternative sounds better and covers more situations.

it's better to do/not to do something it's better to study hard. it's better not to smoke.

2 - “It's better not to know.” you can change 1. "it's better (you) do not know"

For structure with a phrase linked by the preposition "to", [affirmative]: It is better to + (inf.) ... [negative]: It is better not to + (inf.) ... For structure with a subordinate clause linked by the conjunction "that", [affirmative]: It is better that you (do) // one/he/she (does) ... [negative]: It is better that you/they (do not do) // one/he/she (does not do) ... --- Good day. / Que tenha um bom dia.

"It's better not to know" is correct.