When a vaccine is used 3 days before its expiration it still effective?

It depends on the specific vaccine and the storage conditions.

Some vaccines may lose their potency after the expiration date, while others may still be effective for a short time. It is important to follow the manufacturer's instructions for storage and use of vaccines.

In general, it is not advisable to use a vaccine after its expiration date, as there is no guarantee of its effectiveness or safety. If you are unsure about whether or not a vaccine is still effective, it is best to consult with a doctor or pharmacist.