So if cosθ<0, then we're dealing with quadrants 2 and 3 (which are the left most quadrants). Also, since (which means that sine is negative), this means that we're dealing with the lower quadrants 3 and 4.
Take the two regions and find the intersection, you'll find that the only quadrant that they have in common is quadrant 3. So angle θ lies in quadrant 3.