Since, order = 3
Therefore, minimum keys at root, internal node or leaf = 1
And, node pointers = 2
For height = 3,
The number of keys at height 0 is = 1 // root node
The number of keys at height 1 is = 2
Number of keys at height 2 = 4
Number of keys at height 3 = 8
Therefore, total keys = 8 + 4 + 2 + 1 = 15