If you are talking about a 3 day right of recission, there are certain transactions that this applies to, for example properties that require a DRE report. Normal sales do not have this right, although there are usually contractual contingencies that would allow for someone to cancel.
Assuming the agreement was done using a standard and current CA Assoc of Realtors (CAR) purchase agreement form, when a binding contract is formed the buyer has 72 hours from the date that the seller delivers in writing to the buyer the sellers transfer disclosure statement which includes any subsequent and addenda disclosures as long as such disclosures were delivered to the buyer at the same time as the sellers acceptance of the offer. If the seller did not deliver such disclosures at the time of acceptance of the offer than the buyers right to cancel it typically dictated by the terms found in paragraph 14 of the purchase agreement.
This however does not work both ways. The seller cannot cancel during this period. The seller can however cancel the contract if the buyer fails to perform any material aspect of the contract after the seller first gives the buyer a notice to perform. But based on your statement containing the 72 hours language it sounds like you are referring to the buyers right to cancel based on seller disclosures.
This is NOT intended to be legal advice. You should seek an attorney if you believe you need legal advice.