Use the gnomon function to turn the gnomon on or off. The gnomon is a projection of three orthgonal segments parallel to the world x, y, and z axes. The letters x, y, and z appear near the ends of the segments -- a black on white letter means the segment is pointing out of the screen; white on black means into the screen.
A related function is cage3, which turns 3D axes on or off. The cage is a rectangular box surrounding your object, with tick marks on its edges to make your plot a 3D analogue of a typical 2D plot. (Of course, unlike the 2D plot, there is no way you can actually use the ticks around the cage to figure out the coordinates of a point in your picture.) The faces of the cage are at planes set by the limit3 function. The plwf function automatically calls limit3 with reasonable extreme values, but for the pl3tree function you will need to call limit3 yourself. Unlike the 2D limits function, limit3 allows you to set the aspect ratio of the cage; by default cage3 scales your x, y, and z world coordinates to make the edges of the cage appear as equal lengths.