As with all ballistic missiles, gravity takes over after the midcourse phase, and the warhead falls toward its target. Some missiles, like the retired Pershing II, can steer during this part of the flight, called the “terminal phase”; that is not true of the Hwasong-12, whose final destination is determined entirely by course corrections when its engines are still running.
Theoretically, that makes it an easier target.
The SM-3 can intercept a ballistic missile at this point. The United States Navy does not typically disclose the exact positions of its warships, but several Arleigh Burke-class destroyers, which are equipped with SM-3s, are permanently based in the western Pacific. If they were stationed near Guam, they could take a shot at the Hwasong-12s. It’s unclear whether Japan would reposition any of its warships to defend the area around Guam.