Let k be the number of non-leaves.
By the definition of n-tree, the non-leaves must have n child, so there are child of these k non-leaves.
For every vertex (except the root) is the children of some non-leaf and the root is not the child of any vertex, then there are vertices in T.
Since T has 3 levels, then there must be at least 2 non-leaves: the root and at least one of its children that is non-leaf.
Moreover, there are at most n+1 non-leaves: the root and all its children that are non-leaves.
Hence, 2 ≤ k ≤ n+1.