<?php

    /**
     * Implements hook_form_alter().
     */
    function mymodule_form_alter(&$form, DrupalCoreFormFormStateInterface &$form_state, $form_id) {
      $form['field_slideshow_image']['#states'] = [
        'visible' => [
          'select[name="field_selection_display"]' => ['value' => 'slideshow']
        ]
      ];
      $form['field_image']['#states'] = [
        'visible' => [
          'select[name="field_selection_display"]' => ['value' => 'image']
        ]
      ];
      $form['field_video_embed']['#states'] = [
        'visible' => [
          'select[name="field_selection_display"]' => ['value' => 'videoembed']
        ]
      ];
    }