3 months is when you can obtain a conclusive negative test result. There are NO TESTS manufactured, marketed or sold that will give you a conclusive negative test earlier than 3 months.
a pcr is used to moniter the already infected people, the cross contamination can give out false positive more often than an elisa, and even so a undetected viral load has to be followed by an antibody test at the 3 months mork, so pcr is a waste of money so go for an antibody test at 3 months mark