Do you know any manga with this type of triangle ? Preferably a josei over a shoujo (though I will accep them) and also have to be a "serious" triangle not just a random girl that gives a hard time to the main couple for just one chapter.

uwakoi might be what you're looking for

I like turtles

Let's see, there's:

Kagurahime
Cat Street
Tokyo Crazy Paradise
Kimi wa Petto
Hana Yori Dango
Please Save My Earth
Gokkusen
D-ASH

to name a few, but you may already be familiar with these titles.

Outside of a dog a book is a man's bestfriend, inside of a dog it's too dark to read.

Oishii Kankei
Hatsukoi ni Oboreta