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
The Mesomeric Effect
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 :-)