Don't panic, it is still Let's Encrypt.
Let's Encrypt is a free SSL Certificate provider, issuing certificates automatically but only for 3 months.
You could see Let's Encrypt Authority X3 by now new R3 name appears mostly.
Since this kind of changes could be suspicous we can confirm that, SSL Certificates signed by R3 is trustworthy and belongs to Let's Encrypt.
But keep in mind, the SSL Certificates is for:
- Encapsulate your network traffic into encrypted channel driven by mostly TLSv1.2
That means attacker likely cannot see your plaintext passwords, credit card info over the network, however the website still receive as plaintext, so it does not protect you against malicous, fake, scam websites..
- SSL Certificate can prove that the domain is genuine, so protect from Man In The Middle (MITM) attacks.
However does not protect from a special attacks, like one: the link points to a very similar domain name which is also secured with SSL but the website is intended to steal informations, passwords, accounts.
Genuine example domain: mybank.tld
For more information about Let's Encrypt: