FormStateInterface::setValidationHandlers not working properly?
You should add the below method in drupal 8. I hope it should work.
// buildForm

public function buildForm(array $form, FormStateInterface $form_state) {

    $form['company_name'] = array(
        '#type' =&gt; 'textfield',
        '#title' =&gt; $this-&gt;t('Company Name'),
        '#required' =&gt; 'TRUE',
        '#placeholder' =&gt; $this-&gt;t('Your company name'),
    );
    $form['actions']['submit'] = array(
        '#type' =&gt; 'submit',
        '#value' =&gt; $this-&gt;t('Save'),
        '#validate' =&gt; array('::validateForm', '::method1', '::method2'),
    );

    return $form;

}

public function validateForm(array &amp;$form, FormStateInterface $form_state) {
   // do your code stuff..
}

public function method1(array &amp;$form, FormStateInterface $form_state) {
  // do your code stuff..
}

public function method2(array &amp;$form, FormStateInterface $form_state) {
  // do your code stuff..
}