How can I add all three alias attributes in AWS Cognito while setting up Amplify Gen2 Auth?

In Amplify Gen2, you can configure multiple sign-in options for your users by modifying the auth resource definition in your resource.ts file. To allow users to sign in with email, phone number, and username, you need to use the loginWith and userAttributes properties in your auth configuration.

Here's how you can set up your auth resource to enable all three sign-in options:

import { defineAuth } from '@aws-amplify/backend';

export const auth = defineAuth({
  loginWith: {
    email: true,
    phone: true,
    username: true
  },
  userAttributes: {
    email: {
      required: true,
      mutable: true
    },
    phoneNumber: {
      required: true,
      mutable: true
    },
    preferredUsername: {
      required: true,
      mutable: true
    }
  }
});

In this configuration:

1. The loginWith property enables sign-in with email, phone, and username.
2. The userAttributes property defines these attributes as required during sign-up and allows them to be changed later (mutable).

Note that preferredUsername is used instead of just username as it's one of the standard attributes recognized by Cognito and Amplify.

After making these changes, you'll need to push your updated configuration to apply the changes to your Cognito User Pool. Run amplify push in your terminal to deploy the updates.

This approach allows users to sign up with all three attributes and subsequently use any of them for signing in, providing the flexibility you're looking for in your Amplify Gen2 React application.
Sources
User attributes - AWS Amplify Gen 2 Documentation
Fullstack TypeScript: Reintroducing AWS Amplify | Front-End Web & Mobile
Amplify Gen 2, fullname userAttributes not present on sign up | AWS re:Post