When taking a blood drug test, does it detect drug usage for the past 3 months?

It depends: It depends upon what is tested. As you probably know, some drugs are stored in fat cells in your body and can show up in your blood or urine for up to 30 days. There are also tests for metabolites of some drugs, which could be found in your blood for even longer periods of time. It would be unlikely for a blood test to detect drug usage when the last dose was > 30 days ago.