GG.Deals provides excellent info on the price trends of a game, showing not only its historic price but also the 3-month and 12-month lows.

Unfortunately, it is not possible to get alerts for these or any time-based low, with the closest options being to set an alert for a historic low, or manually set the price to the 12 or 3 month low. Ideally, users would be able to select between 3 or 12 months.
Here is a quick mockup of what it could look like. 