That's actually a common misconception. In GPS positioning you have four variables to be determined, your position on the earth (X,Y,Z coordinates) as well as the error of the receiver clock. Because you don't have a high precision atomic clock in your Garmin and the Master Control Station doesn't monitor and adjust the clock in your handheld you need the signal of a fourth satellite to use its high precision clock to calculate exact time differences (distances).

The reason many handheld or car navigation systems also work with three satellites is that they keep one variable, usually the height, fixed in their calculations. You can either use the last known value if a satellite disappears over the horizon or you can just get it from, for example, the navigational maps in your system. When you're driving along some road it generally doesn't matter whether you're 100 meters above or below the road, but it may matter if you're 100 meters to the left or right. This usually doesn't impact the usefulness of the device very much but may allow you to get a fix on your location when you otherwise wouldn't.