"Attorney Review period" can only occur if one or both parties retain an attorney. In this case either attorney can, on behalf of their clients, cancel the contract IN WRITING within the 3 day period with no reason, and without penalty.
Most contracts have wording similar to this right at the beginning:
"This is a legally binding contract that will become final within three business days. During this period you may choose to consult an attorney who can review and cancel the contract. See paragraph on Attorney Review for details."
If either party does not have an attorney and decides to cancel the contract within 3 business days, IMHO, you should do it by certified letter, or at least a notarized letter, as proof that you canceled the contract within the proscribed 3 business day period.
However, I highly recommend that if you enter into a contract of sale as either a buyer or a seller, you retain an attorney. This is a major financial transaction with a multitude of things that can (and do) go wrong along the way.
Not only do attorney's protect your interests and legal rights in the transaction, they are responsible for performing other legally required duties necessary to close on a home. The $$ you spend on this assistance is well worth it.