see this
@sripo root node is also an internal node unless it is the only node in the tree
1. Internal node means non leaf node.
we have for n ary tree
no of leaf node=X(n-1) +1 [By hand shake degree theorem]
where X =no of internal node
So for your question n=3 =>no of leaf node=2*X + 1
it will be [X(2-1) +1 =X+1] for binary tree n=2.