Is it write to write multiple test classes for one source class

Currently I am working in a legacy code. I have introduced a new method to a class. This class does not have a corresponding test class. The method I introduced let's called it isValidState.

This method will only return a boolean considersing some of the states(properties in source class)

Ex

boolean isValidState(){
      return a && b && c;(a ,b,c are properties of the source class)
}

To test this method I am thinking of having 8 test methods.(since there can be 8 valid combinations).

Since source class does not have a test class i have to write a new one which is fine. But in future i might need to add more methods like this to the source class and my Test class will grow heavily and become a very lengthy class.

So to avoid that is it correct to have multiple test classes which will test the functionalities of one source class ? Or is there a better way to have a structured test class/s for one single source class

Thanks in advance