A story where there are three peple in love with each other and at the end there is a happy ending for the three of them together. I have seen hentai with such plot but right now I am searching for shoujo or josei preferably. One more thing: It could be two girls, one boy or two boys, one girl, but now three boys or girls.

The closest possible I can think of is Aisuru Hito.

I enjoyed it It wasn`t exactly what I was seaching for but I had quite a lot of fun. The heroine was one of kind XD
Are there other suggestions?

I can only think of Shinshi Doumei Cross (she wants to stay with two of them) but I don't know if it is exactly what you're looking for. Also maybe Shugo Chara! - we can't tell with whom our heroin will end up in the future but for now they are happy together.

...but that were just wild guesses

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You mean a three-way relationship? I'm also looking for this.

The only one I can think of is the series of comics Humon draws on DeviantArt, "Niels".
http://humon.deviantart.com/gallery/12482917?offset=72

At the end the three characters are to end together. Not simple love triangle at some point of the story and then the heroine/hero chose one.
I have read SC and SDC but they aren`t it. As for Niels`s da, I checked it but its not manga so doesn`t fit the bill >< Still saved few pics