The third house is an apoklima, a decline, which the Latin tradition calls cadent: it falls away from the pivot before it and is reckoned the weakest of the three positions. Its Hellenistic name is the Goddess, Thea.
It is not, however, one of the difficult places, and the reason is geometric. Counting by whole signs the third place stands at a sextile to the first, so it is configured to the rising sign and can see it. Cadence and aversion are two different disadvantages, and the third house has only the first of them. The sixth and twelfth, also cadent, have both, which is why the tradition separates them so sharply from the third and ninth.
The cusp is a quadrant cusp and therefore contested. Whole-sign gives the third house the third sign from the ascendant entire; Equal places it sixty degrees after the ascendant; the quadrant systems divide the arc from the ascendant down to the lower meridian into three parts by rules that do not coincide. Porphyry trisects that arc in ecliptic longitude, Placidus trisects it in time, Regiomontanus and Campanus project from great circles through the horizon and the prime vertical respectively, and Alcabitius trisects the ascendant degree's own semi-arc.
At moderate latitudes the results can differ by ten degrees or more, and near a sign boundary that is enough to change the sign the third house occupies and with it its ruler.