Why the number of equivalence classes for the intersection of an irregular language L2 and a regular languge L3 (with 3 EC) can't be determined?
Assuming you are meaning Nerode equivalence classes, the number of classes of a language is finite iff that language is regular.

We cannot make any statement for the intersection of a regular and a non-regular language. That intersection can non-regular, but also regular.
Similar for the star of a non-regular language. 

Stating that the number of classes is at least one is not very instructive. Isn't that true for all equvalence relations?