How can I get the sizes of 3D vectors in c++ ? : I'm quite new to programming.

I know that if I have a 2D vectors of any size of M by N, 

then I can get the number of the elements in rows, M via myVector.size() 

and I can thus get the number of the elements in columns, N via myVector[0].size()

but if I have a vector of 2D vectors, aka vector< vector< vector< double>>> myVector, of size say K by M by N, then how can I get the sizes K, M, and N? 