The 3-day rule in investing means you should wait at least 3 business days after a substantial drop in a stock’s share price before buying. Stock declines can trigger margin calls causing more stock sales and additional price declines. Typically, stocks continue to drop after declining based on bad news. If you wait three days, you’re likely to benefit in the long term with greater profits and fewer losses. Just remember to review the future growth prospects of the company or industry before buying in after a price drop, to make sure the company is poised to recover.