Look for the banner above on the Amazon Home Page.
Make sure you are signed into your account and click it for a FREE $3 Amazon Instant Video Credit.
Details: You must claim your $3 Amazon Instant Video promotional credit no later than 11:59 p.m. (Pacific Time) on July 18, 2013 or while promotional credits are available. The promotional credit expires at 11:59 p.m. (Pacific Time) on September 6, 2013. The promotional credit may only be used on eligible movie or TV show rentals or purchases from Amazon Instant Video.
And then make sure to claim your reward at Amazon Instant Video. You have 30 days to watch your movie once you rent.