how to create template of user register, login and forget password in drupal 8?
Yes it same as drupal 7.

For example:

We are using page--user--register.tpl.php for registration page template in D7. In D8 changed to twig extension page--user--register.html.twig.

You can enable debug mode in D8 so you can easily see the twig file need to use in your case. You can refer Drupal 8 custom page template to see how to enable debug mode.



Some links how to override the template files.

https://www.drupal.org/node/2521876

https://www.drupal.org/docs/8/theming/twig/working-with-twig-templates

https://www.drupal.org/node/2668886