What happens between the 2 and 3 seasons in Turkish for beginners?

I have seen Turkish for beginners the series on Netflix very often. And again and again the question comes to me on what happens between the 2nd and 3rd season. At the end of the second season, Doris and Metin marry. Lena and Cem get along and Yağmur and Costa go to Greece. However, it is expressly shown that Cem and Ulla are together. But the beginning of season 3 at the Abiparty it is suddenly said Lena had shot down Cem. So my question what happens in between or is that a mistake from the producers?

Metin shows his yarak