What is a 3 day notice

A tenant who does not pay there rent will receive a 3 day notice to pay the rent. If you do not pay the rent within 3 days (excluding holidays and weekends) a landlord will start eviction proceedings with the court.

Well? Since you did not ask this in real estate law, I can not assume you mean rental or real estate law.

A 3 day notice is just what it says,
It is a notice of some sort that you give to someone that notifies them that something will happen in three days.

Normally this is a issue about rent and non payment of that rent.