What exactly is the difference between Web3, blockchain, and cryptocurrency

You can think of blockchain and cryptocurrency as technological implementation details, and Web3 as the communities, businesses, and social relationships that form on top of that technology. A similar analogy would be the original World Wide Web, which could have been construed as a rebrand of the underlying technologies of HTML over HTTP over TCP/IP. Those protocols have served as the foundation for virtually all web content for the last 30 years, but it’d certainly be unfair to describe “the Web” as merely a rebranding of HTML/HTTP/TCP/IP since what happens on top of those protocols is much bigger than just the protocols themselves.

Using blockchain as a database to store personal data in account?

Authentication

Before Web3

  1. Username and password
  2. OAuth protocols(namely OAuth2)

Web3

  1. Wallets that use asymmetric cryptographic algorithm(public key and private key).
  2. Smart contracts enables advances features, like recovery your lost key.
  3. MetaMask.