The anime first season focus on the main four shows which are Re:Zero, Overlord, KONOSUBA, and The Saga of Tanya the Evil. the second season mostly focus on these same four but added Rise of the Shield Hero which appeared in about half the episodes. There was even a small cameo from Cautious Hero: The Hero Is Overpowered but Overly Cautious.
There isn’t much detail about the third season yet, just as the 2nd season ended they let us know we can look forward to more. You can read our Hidden Gems Review for Isekai Quartet Here.