Where is coil 3 on a 2003 Ford Expedition?

A 2003 Ford Expedition doesn't have numbered ignition coils in the way you might be thinking. It uses individual coil-on-plug (COP) ignition system. Each spark plug has its own ignition coil directly mounted on top of it. There isn't a "coil 3" in the sense of a separate component.

To find the coil for a specific cylinder (assuming you meant cylinder 3), you'll need to consult a repair manual or online diagrams specific to the 2003 Ford Expedition's engine (either the 4.6L or 5.4L V8). These diagrams will show the cylinder firing order and the location of each coil on the engine.