If the field strength is not low enough between the magnets all you need to do is alternate the poles of the magnets (N / S / N / S....).
Note that the 3120 only responds to positive gauss (whether that's N or S I dunno). I'd suggest picking one of the devices (3x30 or 3x75) that latches between N and S instead so you're certain you can get eight evenly spaced events when you're using magnets with their poles alternating.