Stability of tertiary carbocation
Your teacher is correct, that 


  a carbocation connected to 3 cyclopropane rings is among the most stable carbocations.


This because of various electrical effects, such as:


The Inductive Effect
Resonance
The Mesomeric Effect
Electromeric Effect
Hyperconjugation


Here's a link for you, to study them: Click Here.
Please make sure that you learn this correctly and from standard resources, in order to avoid confusion.

There may be a little confusion between mesomeric effect and resonance effect, so here's another link just to avoid confusion. : refer to the answer by Maria Dar

Please, do ask your teacher to help you understand this topic better :-)