Description states

More examples in the test cases.

This refers to the main tests. You can print the input to see what that is:

function common(a,b,c) {
  console.log(`a = ${a}`);
  console.log(`b = ${b}`);
  console.log(`c = ${c}`);
}

Both numbers are False when I try them in my code (that passed all the tests), so I guess it is not expected to be True. Maybe there is something else wrong with your code...

Random tests are susceptible to input mutation - this is a kata issue. To pass the kata, don't mutate the input array.