Prongs are "branches" that come off the stem. It's all about telling the age of the root, and a three prong plant is most always at least three years old, which makes it legal. When you dig the root, there will be bud scars on the neck of the root, one for every year the plant sent up a shoot....like counting rings on a tree stump.
When you dig a three prong plant, you double check the bud scars to verify age. If it doesn't have three, you re-bury it or leave the top attached while in the woods to verify that you dug according to the book. If it has easily identifiable bud scars of three or more, you may discard the top. After planting any berries of course.
