In scene 280, the three fronds are connected together:
The Third Frond is slid into place with the other two Fronds, filling in the 1/3 vacancy.
In scene 283, it's observed that once the three fronds are interconnected, a Hexagon is seen to grow between them:
The SET OF 3 FRONDS quietly churns in the cave. A 6 INCH HEXAGON is growing from its center where the Leaves of each Frond meet.
Shortly, in scene 284, the Hexagon is seen to grow of it's own accord:
The Set of 3 Fronds sits alone. The Hexagon has grown twice as wide and to a height of 10 inches.