You can definitely make it. As mentioned by other posters, there's a sterile walkway that connects Terminal 3 with the International Terminal gates that UA uses, so there's no need to go through TSA again. Having said that, the one suggestion I'd make is that you walk briskly to the gate for the Frankfurt flight. You don't mention which class of service you're in (First, Business or Economy) or if you have status, but if you're in Economy you want to get to the Frankfurt gate as early as you can. The boarding, and then competition for overhead bin space might get chaotic if it's a full flight.