The deepest rule in Renju: a three only counts if it can actually become a straight four — and the completing point must not itself be forbidden. Here black's center column three is alive, but the horizontal 'three' has both of its straight-four points on forbidden intersections. So the horizontal three is FALSE — it does not count. Click the star point: is it a double three? The upshot is practical, not academic: as black, verify a fork before you trust it; as white, remember that some of black's scariest crossings are already dead on arrival.
Finish this step to move on.