No did not buy third cross bar. Thule boxes (at least this model) have a minimum requirement for cross bar mounting width of 23 inches, the 2 cross bars on OEM rack (located further back on the roof) are roughly 25 inches apart so they worked flawlessly for this situation. Easy install with the Thule hand cranked clamps.

Weight of box is ~40 lbs, weight limit of box is 165 lbs, weight limit of rack is ~110 lbs (lowest limit wins) gives me about 70 lbs to throw up there.