There are different ways to create the path along which the object and camera or view will move.
You can use existing line features, such as roads that are draped over a landscape, as the path. By selecting a particular line segment, the options Move Layer along Path and Camera Flyby from Path (Create Flyby from Path in ArcMap) will become enabled on the Animation toolbar.
If you want an object to follow a 3D path, such as a plane following its trajectory, you can create a 3D line features using the Editing tools in ArcScene.
If you want an object to follow a straight line that doesn't take into account the surface over which it is traveling, such as a plane flying above a landscape at a constant height, you can create a 2D line feature in ArcMap using the Editing tools, then apply an offset to the line so it is not obscured by other layers in the display.
If you want to set specific z-values for your line features, you can create 3D features for use as the path.
The 3D Graphics toolbar in ArcScene allows you to make 3D line graphics. These graphics will draw based on objects already in the view. Using the toolbar, you can digitize a line to use as a path along which to move a layer or from which to create a camera flyby. You may need to apply an offset to the line graphic so it is not obscured by the surface when the animation is played.
Using the Draw toolbar in ArcMap, you can create a line graphic to use as a path. Once you've added a graphic to your map, you can move it, resize it, change its color or symbol, and so on.