register - 3

// ...
class Register extends Component {
  constructor() {
    super();
    this.changeEmail = event => this.props.onChangeEmail(event.target.value);
    this.changePassword = event => this.props.onChangePassword(event.target.value);
    this.changeUsername = event => this.props.onChangeUsername(event.target.value);
    this.submitForm = (username, email, password) => event => {
      event.preventDefault();
      this.props.onSubmit(username, email, password);
    }
  }

// ...