classA.js

((function (global) {
  class A {
    constructor() {
       console.log('soy clase A');
    }
  }
  global.A = A;
}(this));

classB.js

((function (global) {
  class B {
    constructor() {
       console.log('soy clase A');
    }
  }
  global.B = B;
}(this));

main.js

((function (global) {
  const A = global.A;
  const B = global.B;
  class Main {
    constructor() {
      this.instanciaA = new A();
      this.instanciaB = new B();
    }
  }
  new Main();
}(this));