You're doing the one hour glucose test. The three hour test is usually done if you "fail" the first or if there is an unusual/concerning result. The 3 hour is a fasting test. No eating between 8 and 12 hours prior to drinking the drink and then they draw your blood at the first hour mark after you drink, then the second hour mark after, and finally the third hour mark after and look at the results together. It gives them a better, more measurable comparison of how your body is dealing with your blood sugar. At least, that's my understanding.
If you fail the hour test you have to go back for a three test